Lopburi is one of the oldest cities in Thailand, a former capital and the second capital after Ayutthaya was established in 1350. It was abandoned after King Narai passed away in 1688, but parts were restored in 1856 by King Mongkut (King Rama IV) and in 1864 it was made the summer capital.
Lopburi has been an important part of the Khmer Empire, later a part of Ayutthaya kingdom, and Ayutthaya's second capital under the reign of King Narai the Great, who used to spend eight months of the year in Lopburi. Later on, King Mongkut of the Bangkokian Chakri Dynasty resided here. There are remains from almost all periods of Thai history.
Lopburi is famous for the hundreds of crab-eating macaques that overrun the Old Town, especially in the area around Phra Prang Sam Yot and Phra Kaan Shrine, and there's even a monkey temple/amusement park where you can buy snacks to feed to them.
Keep an eye out for monkeys hanging from trees and wires and sitting on roofs and ledges, and be aware that they have some unpleasant bad habits including defecating on unsuspecting pedestrians from their overhead perches, jumping on people to snatch food and stealing bags that they suspect may contain something edible.
At night nothing much is going on in the Old Town, thus the street dogs consider everybody running around after midnight very suspicious. While most of them will just look at you, some might bark, run behind you and jump at you. However, whilst common at night it is very rare during the day.
From Ayutthaya: local buses run every 20 min, take around 2 hr, and cost ฿35.
From Bangkok: air-conditioned buses leave every 20 min and take either 2.5 hr and cost ฿117, or take 3 hr and cost ฿96. Take the air-conditioned bus from the Bangkok Bus Terminal (Mo Chit 2) which departs every day 05:00–20:30 every half hour. It costs ฿80 per person.
NOTE: Currently the bus is not running, but a van service from Mo Chit is available.
From Kanchanaburi: it's necessary to take a local bus to Suphanburi taking 2 hr and costing ฿50, then another from there to Lopburi taking 3 hr and costing ฿52.
From Sukhothai: take a bus to Phitsanulok and then to Nakhon Sawan.
From Bangkok, air-conditioned vans leave from Victory Monument, take about 2 hours and cost 120 baht. There are multiple van services in the area, if the timing of one service is not convenient then try another.
Air-conditioned vans also leave from the main Mo Chit (northern) bus station for the same price. The last minivan normally departs around 18:00.
From Saraburi, a minibus from the bus station will cost B60.
It comes in through two large roundabouts, the second one appears to be the main minibus stop.
Trains from/to Bangkok's main Hualamphong station take about 3.5 hours. Take the Northern Line from Hua Lamphong Railway Station everyday, many rounds per day. Fares range from 13 baht in ordinary class (which is fine) to 370 baht in aircon coach with a meal.(March 2014) The expensive train is also a little faster.
Trains from/to Ayutthaya take about one hour.For more info on Thai rail travel, see 
The blue local bus (8 baht) circles constantly between the bus station about 2km from the town centre, passing Phra Kahn Shrine, going south on Thanon Sorasak, and ending up in front of the TAT office on Phraya Kamuad Rd.
Ban Vichayen, Narai Maharat Road. The remains of Constantine Phaulkon's residence, built in the reign of King Narai the Great. Only the outer walls of the three main buildings remain, in a small grassy area. Daily 08:30-16:00. 30 baht.
Phra Kahn Shrine, Narai Maharat Road. The site of a small shrine, the remains of a Khmer prang, a few stalls and lots of monkeys. The stalls sell offerings to be dedicated at the shrine, and food and drink. The monkeys eat the food, drink, offerings and anything else going. Good for a few photos. There are signs warning of purse-grabbing by the monkeys, but they appear docile if not provoked. 50 Baht.
Phra Narai Ratchanivet or King Narai's Palace, entrance on Thanon Sorasak on east wall. Built in 1677 by French, Italian and Portuguese engineers, the palace was used by King Narai to host receptions for foreign envoys. Restored in 1856 by King Mongkut, it was converted ino a museum in 1924. The palace grounds consists of the remains of various buildings in an enclosed park, with the central palace serving as the Somdet Phra Narai Museum, which houses prehistoric exhibits, along with Buddha images of Dvaravati, Lopburi and Khmer styles; and King Mongkut's bedroom in its entirety. Open Wed-Sun 8:30-16:00, closed Mon-Tue and national holidays. Entrance fee - foreigners 150 baht and Thais 30 baht.
Phra Prang Sam Yot is a Khmer-style temple known for its three linked towers. Entrance fee - foreigners 50 baht and Thais 10 baht.
Wat Phra Phutthabat, 17km south-east of Lopburi. Take any Saraburi bus (#104) which leave the main bus station every 20 mins, take 30 mins to get to the side road 1km from the wat, and costs 21 baht.
Built in the 13th Century, Wat Phra Sri Rattanamahathat is one of the town's most important monasteries; visitors can view a bas relief illustrating the Buddha's life on the central prang. No monkeys. Entrance fee - foreigners 50 baht and Thais 10 baht.
A viharn belonging to a working wat, Wat Sao Thong Thong, on Rue De France, also has a small amulet market in the grounds. Previously used as a Christian chapel and a mosque, it has now been restored and features a large Buddha figure, with several smaller Lopuri-era Buddhas in wall niches. No charge.
Evenings, a lot of street food stalls are set up on a road in front of railway station. The street vendors in the Old Town are very nice and have all kinds of tasty things. Don't be afraid to stop and check them out.
Bua Luang - in the New Town, about 6 km from Old City. 46/1 Moo 3, Tasala A. Muang. Tel +66 36-614-227/8/9/30. Cash only.
Louis Steakhouse - on Phahon Yothin east of the large traffic circle 1/2 km or so from Big C under the pedestrian overpass. A great restaurant owned by a fellow from Belgium. A great change if you are looking for something a little different from normal Thai food.
New World Steak House - Great if you like good English cuisine. Run by Barry and Noi, an Englishman and his Thai wife. This spot is just west of Sakal (the huge centre of town with the fountains) just to your left before you cross a bridge, at the lights (look for a rather large hotel next to it). The prices are higher than typical Thai food, but the steaks are huge, the Shepherd's pie is awesome.
School Milk, Next door to Noom Guesthouse. Located next door to Noom Guesthouse. Large variety of Thai/Western food, snacks, shakes and coffees. Excellent quality and huge portions for low prices. Caters to mostly students and young people. Update March 2013: portions are definitely not huge and unfortunately the quality is poor. Food was served lukewarm and staff seem more interested in their own lunch rather than the customers'.edit
White House is located just behind (north of) the tourism office (TAT). Romantic western architecture with a beautiful yard and second floor, offers nice food. Crab meat, fried rice and red curry is very good. The owner, Mr Piak, speaks English and gives good advice.
You might find the nightlife in Lopburi fairly quiet for a town of its size but there are places for a drink in the evening. Old Town has a few curbside bars, which are excellent for those who are still new to Thailand, as there are usually some foreigners. There is also a small club (look for the large "Ben More" sign) next to a local park near the train station in the Old Town, but it is a little pricier than average.
The centre of town has a variety of places, from hole-in-the-wall local dives, to "The Bank", a disco that is frequented by Lopburi's younger crowd (not recommended unless you know your way around well; foreigners are rare in the Bank). Uptown has few drinking establishments on the main road, but there are a variety of karaoke bars and such down the back roads. Some of these out-of-the-way places are decent for a drink and some offer female company (also not recommended for the newcomers).
Butterfly Bar, Phayakamjad Rd ((across street from Narai Palace)). 12:00. Nice little streetside bar with beer, whiskey and food. Right across the street from the Narai Palace (east gate). Gung and Steve are great hosts and the bar stays open until there is no one remaining. There are usually a few westerners.50 baht. edit
Hotels in the Old Town offer generally similar medium scale standards for low 140-500 baht range prices. The monkeys run around freely but usually stay in just one small area. Depending on your preference you can choose a place with lots of monkeys running (and hanging) around, or opt for somewhere with low or no monkey presence.
Places with lots of monkeys:
Lopburi City Hotel - probably the best of the hotels within the monkey area, and enclosed in a big "cage" that keeps the monkeys out, so you can open the windows. All rooms are air-con. Prices from 300 baht.
Muang Tong Hotel - this is the least likable hotel in the monkey area. It's not enclosed in a "cage", so opening the windows isn't a good idea. However, it does have the best view of the monkey area and the Phra Prang Sam Yot Temple. Rooms have Thai-style bathrooms with squat toilets.
Sri Indra Hotel - enclosed in a big "cage" that keeps the monkeys out, so you can open the windows. The rooms are neat and clean, but don't expect more. Prices from 200 baht.
Places with few monkeys:
The newly opened "Ruan Thai Guest House" is located in a renovated wooden house behind the King Narai Palace, close to the river and overlooking the Wat Chering temple. Spacious upstairs rooms with gleaming hardwood floors and balconies; with or without A/C, WiFI. Clean, cheap restaurant downstairs. Run by enthousiastic women. From 350 to 500 baht.
Lopburi Asia Hotel - very close to the King Narai Palace. Rooms are low to medium standard. Prices from 270 baht.
Nett Hotel - good location, with a small food market in front, and no monkeys running around. Rooms range from medium standard to very likable medium standard (after recent tiling). Prices from 180 baht.
Noom Guest House, 15-17 Phayakamjad Rd, Thahin Maung www.noomguesthouse.com - has fan rooms, also offers motorcycle rental and rock climbing. Serves English breakfast, 08:00-11:30. Single room 200 bahts. Double bed room with bathroom 350 bahts.
Suphon Phong Hotel - has only two good points: its location (very close to the train station and to Wat Phra Sri Ratanamahatat) and the price, from 140 baht.