I can't recommend Ko Chang more highly. It is the perfect antitode to the craziness (and humidity) of Bangkok. We went in November '04 and it was amazing. It is so unspoilt, with no buildings allowed to be higher than a palm tree, so when you are swimming in the sea, you can barely see buildings, just lush vegitation. You have to get a ferry there from Trat, which takes about 40 mins.
Once you are there I'd recommend going to Bang Bao, a small fishing village on the island for delicious seafood (squid stuffed with mince pork and chilli...mmmmm). Nightlife was very low key and chilled out, but we did find an amazing beach bar which had a barbeque most nights with the hugest shrimps I've ever seen. It was next to our hotel (whose name escapes me at the moment, I'll have to try and find it). The hotel itself was lovely, with its own beach, pool, and sumptuous breakfasts included in the price. It was so cheap too, for such luxury, prices then were about €40 a night. Oh and definitely agree about having spa treatments, most hotels have spas, and they are fabulous...
Obviously it's been three years since we were there, so I'm sure it's a bit more touristy than it was, but it was very undeveloped then. I really hope it hasn't changed too much. We want to go back as soon as possible before it becomes too developed. It really is the most magical place...writing this has made me want to book tickets back there!
Whatever you choose, I hope you have an amazing time.
I've added a picture we took at Bang Bao one evening.