Make sure you check out Bangkok for a few days. A canal tour on a river taxi was great. try to find a supped up one for a more fun ride. We stayed in Bangkok at both ends of our trip and really enjoyed it even more the second part as we werent trying to go and see everyting but just enjoyed the city. Such nice hotels available so inexpensive.
As for beaches, I like Phuket but would recommend staying down on Kata beach or Karon beach. Avoid Patong beach if possible. There are a lot of good resorts all over that area. Marriott has nice resorts in the area.
Krabi side is also great if you are looking for even more laid back/rural setting.
One of the best things we did was book a private boat tour out to Phi Phi as opposed the bigger tour boats. Well worth the little extra expense.