Awaji Island (淡路島 Awajishima in Japanese) is a not-terribly-large island - about the same size as Singapore - that marks the eastern boundary of the Seto Inland Sea of Japan. Thanks to a set of new bridges and a cross-island expressway, most visitors just zip through on their way from Honshu to Shikoku.
Awajishima has some claim to being the oldest settled area in Japan; the Kojiki mentions it under the name "Onokoroshima" and burial mounds (kofun) dating back thousands of years have been found on the island. The ningyo joruri puppet theater, which has evolved into bunraku, seems to originate from Awajishima.
Awajishima made a highly unusual but brief appearance on the world stage as the epicenter of the Great Hanshin Earthquake of 1995 that killed over 6000 people. However, Awajishima was (and remains) far less built up than the suburbs of Kobe across the bay, which took the brunt of the damage. It was also the island where the England soccer team stayed during the World Cup in 2002.
The southern tip lies a mere kilometer off the coast of Shikoku, and a bridge now straddles the Naruto Strait, famed for the whirlpools that form as the tide flows in and out. The very name "Awaji" means "road to Awa", the former name of current Shikoku prefecture of Tokushima.
At the other end of the island, some 50 kilometers away, the northern tip is not far from the port city of Kobe on Honshu, and the immense 3.5 km Akashi Kaikyo Bridge — the world's longest — now connects Awaji to the mainland. Politically (and in geographic terms somewhat oddly), despite its proximity to Shikoku, Awaji is a part of Honshu's Hyogo prefecture.
Highway buses connect Kobe Airport and Sumoto four times daily (two hours, ¥2000).
A highspeed ferry used to run between Sumoto and Kansai International Airport but this service stopped in 2007.
The inter-island expressway will get you from Akashi to Naruto, but it isn't advised unless you're willing to part with ¥5000 in tolls. Furthermore, signage is in Japanese and may be incomprehensible to a foreigner.
There are no direct train services to Awaji Island. Highway buses run directly from major train stations, such as Shin-Kobe on the shinkansen (¥1800 to Sumoto), and Osaka and Sannomiya stations on the regular JR line (¥2300 and ¥1800 to Sumoto, respectively). From Shin-Osaka station you must either take a local train one stop to Osaka station, or remain on the shinkansen to Shin-Kobe, to transfer to the bus.
Buses to Awaji Island are not valid with the Japan Rail Pass. Tickets can be purchased from "Midori-no-Madoguchi" locations at each station.
Even cheaper and more scenic, but available for the northern crossing only, are ferries that cross from Akashi to Iwaya for a mere ¥320 on the slow boat (all of 24 minutes) or ¥450 for the fast boat (a zippy 13 minutes) via the Jenova Line  located south of JR Akashi Station.
Public transport is limited to very occasional buses. Awaji Kotsu posts the bus schedule  (only available in Japanese). Unusually for Japan, there are no trains on the island. If you don't have your own set of wheels, hitchhiking is a viable option.
Very little evidence of Awaji's history remains though, and today's Awajishima is a typically Japanese densely populated but still rural area, known primarily for its onions. The current total population hovers around 150,000, and (unlike most rural areas in Japan) is slowly on the rise due to the improved connections to the mainland, and these days Awaji's most impressive structures are its bridges.
Scattered here and there are a number of herb and biwa (loquat) farms. The southern coast, however, is essentially one long semi-urban sprawl filled with the
Awaji-shima Koen- Located on the northern part of the island just off of the freeway is a giant park full of open fields, fountains, playgrounds, slides and walking paths. Free.
Awaji Hanasajiki- Located in the central/northern part of the island, this park is not to be missed during spring bloom. The park is filled with flowers and a breathtaking view. Free.
Awaji Highway Oasis- A highway stop on the northern part of the island where you can buy souvenirs, eat and ride the ever famous ferris wheel.
Hokudan Earthquake Museum- The museum memorializes the damage sustained from the famous Kobe earthquake of 1995. Aside from the earthquake simulator that you can ride, it has preserved a building broken in half, cracks in the ground and walls and other damage to help one appreciate the extent of the quake. Small entrance fee.
Awaji Yumebutai- Located right next to the Westin hotel, endless steps take one on an endless flower adventure. Entrance is free and it can be accessed through the hotel.
Akashi-Kaikyo National Government Park- Located right next door to the Awaji Yumebutai, this park has flowers, flower sculptures, a large playground, a train that goes around the grounds and a pond where one can rent a paddle boat that looks like a swan. Small entrance fee.
Yuzuhara Dam- Located in Minami (southern) Awaji is a beautiful damn and park. A great place for viewing cherry blossoms in the spring (hanami).
Onions ("tamanegi")- During the summer, you'll find onions at every stand along the road. Some places sell large bags for around 200 yen. The onions are sweet and are even better than Maui Onions.
Awaji beef (Awaji gyu)- Cows are raised deep in the mountains and fed special diets and (supposedly) massaged daily to create a meat with a perfect, fatty marble. Awaji-gyu is much more exquisite and delicious than its counter-part, Kobe beef.
Awaji Coffee Milk- This can be found at any grocery store or convenience store (konbini) on the island, and is a rich blend of creamy milk coffee that is not to be missed. You can even visit the factory in Minami Awaji (the southern part of the island).
Squid (Ika) Senbei- This can be found at the Awaji Highway Oasis (large souvenir store on the northern tip of the highway), or at the factory itself located in central Awaji.
Takoyaki- Fried dough balls/dumplings filled with octopus are famous in the Kansai area.
Okonomiyaki- The "Japanese pancake" can be topped with an array of toppings (bacon, onions, squid, etc.) and is also famous in the Kansai area.
Awaji has a scattering of ryokan and minshuku, concentrated in the hot spring areas. There are also a number of campsites, especially on the less populated western coast.