Fira (also called Thera or Thira) is the main town of Santorini island. Though it has little except the museum, the cable car and the few bars and clubs, it is a good base to use as there are buses from many other parts of the island.
Every day during the summer is a bustling metropolis as cruise ships from all over the Mediterranean and Aegean seas descend. As a result prices in Thira (and Santorini generally) tend to be a little higher than other Greek Islands.
Information on how to get into Santorini can be found here.
From Santorini airport: by taxi, or shuttle bus 
From the old port, where cruise-ships arrive: by cable road, donkey or by foot.
Fron the new port (Athinios), where all the ferries arrive: by taxi (expensive) or by local bus (2€, can be crowded), or by shuttle bus , value for money
The main bus station of Santorini is in Fira, where you can catch a bus going almost anywhere on the island, usually hourly (every 30 minutes on 6/29/2009). The schedules are posted here daily. Up the stairs behind the bus station and across a parking lot to the west is the main taxi stand.
A public restroom is located at the southern end of the taxi stand. There is a crazed woman who confiscates the toilet paper from the toilets in the mornings, and then accosts patrons as they enter, demanding 50 euro cents for a couple of sheets of toilet paper. She follows people into the restroom and continues to harass them while they use the facilities. Bring your own napkins, and try to ignore her.
Another great way to get around the (small) island is by hiring a quad bike (usually named "ATV") from numerous locations in the main tourist villages. for €20 a day, it is better value than taxis and more fun than a bus.
Definitely pay a visit to the Museum of Prehistoric Thira, where the story of Akrotiri and other ancient settlements is vividly told through a stunning collection of Neolithic and Bronze Age artifacts. (free for students) Special ticketing package for the Archaeological Museum the Museum of Prehistoric Thera and the Archaeological Sites of Akrotiri and Ancient Thera: € 8 (reduced: € 4).
There's actually not a lot to do in Fira. Apart from the few museums and craft studios (which are open fairly late), travelers seeking unique experiences unobtainable elsewhere will be disappointed.
Stroll through the shopping district (for overpriced jewelry, designer clothing and handbags) and go to the numerous (American-styled) discos and bars.
Watch a sunset over dinner.
Walk along Agiou Mina street. This street goes through Fira part-way down the cliff, from the south part of town all the way to the cable car. It provides the stunning views of Fira that you came to see. You can walk either north or soth, but walking north has the advantage that the main part of Fira is in front of you. To get there, from the the main cathedral, walk with the sea to your right, just left of Hotel Atlantis, about 50 m. Walk around 1500 BC restaurant, and you'll be on the path. Just be careful of the donkey poop!
Go to the Old port. Whoever said "Getting there is half the fun" was likely referring to the Old Port. Or, in this case, getting there is 99% of the fun. There is almost nothing to do at the Old Port (except perhaps take a couple of pictures from sea level). But the ride down and up can be fun. Your choices are the cable car (€4 one way), donkey ride (€5 per person, plus a €5 "tip" for your "guide"). Walking is possible but not very pleasant, thanks to the donkey excrement covering the path; moreover, donkey drivers are bent on making life difficult for tourists who refuse their business, so donkeys are not at all encouraged to avoid people while walking up or down. If you want to avoid the smell and the scene just take the cable car; even though the line may seem long sometimes (especially when people from the ferries arrive and leave), it moves pretty fast (36 people every 6-7 minutes or so). Some local mule guides may also try to cheat you by demanding a payment for walking down, they will try to sell you a small piece of paper that says ticket without any numbers on it. Ignore these people, the road is free. They will also try to convince you that the walk down takes one hour; it's a lie, it doesn't take more than 20 minutes, even zig-zagging to avoid the donkeys and their poo.
Get out of the tourist section. The tourist part of Fira is surprisingly small; a couple minutes walk away from the sea brings you out of the tourist area. While the architecture here is not stunning, it certainly is different than you will see elsewhere. Plus, you'll have a chance to marvel at the fact that even though Fira land prices are very high, there are still empty lots in the middle of the city.
In the early evening the passengers of the cruise ships return to their vessels and cafes become much more relaxed.
Akropolis, odos 25 martiou (on the main road to the town of Oia). The price of the dishes is lower than elsewhere in town, and that for outstanding food! The lady in charge is a superb cook and very friendly. Yet tourists didn't find the way to this lovely place, the locals know about the quality of this great restaurant.edit
Koukoumavlos Restaurant, ☎ 22860 23807. Restaurant Koukoumavlos, by chef Nikos Pouliasis, serves modern Greek and Mediterranean cuisine. Taste some of his favorite creations: Santorinian fava with smoked trout, salmon tartar in mandarin sauce and roasted almonds. Lobster and monkfish terrine with anchovy caviar sour cream in forest fruit tea sauce. Grilled beef fillet with potato puree flavored with bergamot, summer truffle sauce and parmesan ice cream. Tiramisu with smoked eggplant mousse on a base of almonds in green bitter orange sauce.edit
Nikolas, Erythrou Stavrou (Head straight up the pedestrian mall (Danezi) and take the first right), ☎ 2286024869. A local eatery preserving traditional Hellenic cuisine. No English menu is provided, but the staff is efficient in explaining the Greek menu board and provide suggestions if necessary. Service is a departure from the overly warm receptions with most tourist traps as it is quick, efficient, and sufficiently friendly. Must tries include the Horiatiki (Greek) salad, moussaka, beef stew. Ask for "horta", a plate of simple boiled greenery ordered by the locals and not on the menu as it is both time-consuming and difficult to prepare on a mass-scale. Though the dish may not look overly appetizing when it arrives at the table, sprinkle on some fresh lemon juice from the provided wedge and add some salt to bring the traditional veggie dish alive.Appetizer: 4 Euro, Mains 8-14 Euros. edit
Ouzeri, (Located in the center of Fira at the Fabrika shopping center which is next to the Cathedral), ☎ 22860 21566. George and Iordana, the owners, welcome warmly all their guests. You will find a great variety of traditional Greek dishes using the local products of Santorini, Greek salad, green salad with Chorotyri, Mousaka, Ouzo meatballs, Soutzoukakia.edit
Santo Mythos, (Located two blocks from Theotokopoulou square on M. Danezi street), ☎ 22860 24346. Offers a wide range of cheap, tasty food including salads, sandwiches, moussaka, a variety of gyros, pizza, spaghetti, omelets, and both savory and dessert crepes. Great for travelers on a budget and the service is relatively prompt by comparison to many of the other restaurants in Fira. Gyros are €1.70, Greek Salads are €4.50. They also deliver from 10:00 - 24:00.edit
Shinx Resturant, (Center of Fira in front of the Cathedral). Restaurant Sphinx has been accommodated in a restored mansion in Fira since 1991. Its unique view over the Caldera of Santorini becomes more breathtaking during the exquisite sunsets just in front of you. Chef and owner George Psychas creates Mediterranean cuisine which showcases the fresh local produce and Aegean Sea fish. Relaxing environment, extensive wine selection.edit
Two Brothers Bar, An internationally famed watering hole, catering to the tourist party crowd.
Bonjour Café, (Central Square of Fira), ☎ 22860 23744. Modern café bar open all day offering variety of coffees, juices, soft drinks cocktails and snacks. Open from early in the morning until late after midnight.edit
The Highlander, a Scottish bar, Happy Hour from 9-10:30 PM
Koo, cool nightclub to hang out after drinking at the other bars and to close out the night..
Adamant Suites, ☎ +30 22860 23432, . Adamant suites is a small private complex consisting of six private suites, all different in size and shape.edit
Aria Suites, ☎ +30 22860-28650, . Boutique hotel with a view of the volcano and Acrotiri.edit
Athina Suites, ☎ +30 22860 24910, . A short walk from downtown. Mounted on a rock top. Offers a panoramic view of the Caldera cliff and the volcano. edit
Cliff Side suites. (tel: +30 22860 22141 fax: +30 22860 22187) 23 rooms and suites with all four star conveniences. Respecting the Aegean Architectural style, the complex promotes the design and comfort of the locality, combined with modern colors. Includes on-the-edge-of-the-rim swimming pool with views of the caldera plus spa services and outdoor heated Jacuzzi.
Mill Houses Suites, ☎ +30 2286027117 (fax: +30 2286025400), . The exterior architecture of Mill Houses Suites in harmony with the geological peculiarities of the landscape.edit
Panorama Studios & Suites, ☎ +(0030) 2286025750 (fax: +(0030) 2286025751). The Studios are located in the center of Fira on the Caldera edge all rooms have balcony facing the sea and the volcano. 12 studios, one Grand Suite, one honeymoon suite and 2 Petite Suitesedit
Summer Time Pension, ☎ +30 22860 24313, . 100 meters from the center of the town.€83-110. edit
Villa Evgenia, ☎ +30 22860 22575, . 300m from town center.€70-120. edit
Anteliz Hotel and Suites, Markou Nomikou 89, ☎ +30 22860 28842, . Four star hotel on the top of the cliff ridge along the Caldera path, overlooking the Caldera Gulf, the Volcano and the settlements of Oia and Akrotiri.edit
Villa Stella, ☎ +30 22860 23517, . Family-owned hotel in a quiet district 7 minutes from the centre. Clean rooms with sea views.€35-40. edit
Hotel Mylos, ☎ +30 22860 23884, . Hotel Mylos is located in Firostefani,350m above the sea level offering to the quests a spectacular view towards,the Volcano of Santorini and the popular Breathtaking Sunset in Santorini.edit
Astraea House, Markou Nomikou (Fira), ☎ +30 6977122887, . checkin: 15:00; checkout: 12:00. An original captain house , with Amazing view!from 240. edit
Easy internet cafe with 20PC minimum charge 1€. Next to taxi station. A nice and calm greek internetcafe to see your e-mail, web surf, chat. They also have Wi-Fi with 2Mbps bandwith. You will find it from the orange color.