I was there for two weeks in February. You are going to love it. It is really easy to get around if you only speak English, and the Thai people are welcoming and lovely.
I'm glad that it looks like Sukhothai is on your agenda. I spent 4 days there, which was plenty of time, but you probably need at least two days. The town itself is simply a typical rural Thai town (which will be nice after Bangkok!), but the old part of town with the ruins are atmospheric and beautiful. (And will totally get you geared up for Cambodia to see Angkor Wat and the other temples.) We stayed here: http://www.lotus-village.com/ and I would definitely recommend it. You get a separate bungalow decorated in traditional Thai style including breakfast (coconut yogurt- yum). Go to Dream Cafe for food and check out Kitty House for drinks (everything is decorated with Hello Kitty)...
Haven't been to any of the islands where you're headed, though I've heard good things about Ko Tao for diving (if you don't already have your certificates, you can get them in Thailand, but you'd probably need to spend more than four days in Ko Tao to do it). I can vouch for Ko Chang (the marine park, the one near Cambodia) and I have friends that are partial to Ko Samui (although Samui has gotten more touristy and developed a la Phuket). It really depends on what kind of beach/island experience you are looking for though.