I go to Koh Chang frequently, last weekend I stayed at Nirvana near Bang Bao Village. It's a great place to relax. The main mongering bars are at White Sands Beach, across from the Chonburi Resort. It's possible to find some gems but the overall quality of the girls is not great so you may want to bring your own if you're particular. There are bars scattered along the western side of the island, the eastern side is mostly undeveloped. There's a nice waterfall not too far from the Amari.
The island itself is the second largest in the kingdom. Accommodations in all price ranges, some great high-end resorts. Also possible to get a room with a fan for 200 or a bungalow on the beach. Some excellent seafood restaurants, or expensive restaurants serving mediocre farang food.
Highly recommended. but if you're going for the bar scene and nightlife you'll be disappointed.
Roads are dangerous at the northern and southern ends of the islands, hairpin and s-turns on steep inclines. Extremely dangerous at night, but roads are ok between White Sands Beach and Kai Kae Beach. I ride there by motorcycle. Ferry crosses the channel once an hour until 6 and the trip takes 40 minutes to an hour, depending on where you cross. One-way fare is 100-120.
The Amari is nice but expensive. For the same price you can get a Villa room with small private pool with 10 foot wall for privacy at the Dusit Princess. I've also stayed at the Tropicana, Nirvana, Aana, Sea View, Kacha, Ban Pu, and a few others. Night scene is mostly concentrated at White Sands Beach but other beaches are better so would recommend you stay elsewhere if you're interested in the beach scene.