Having lived in Thailand for quite a while and having been to the places you discuss, I'll offer the following: Koh Samui and Koh Phangan are not the best places to visit. Someone else suggested the Phi Phi islands and I second that. Koh Tao and Krabi are other places to consider. Some great places to see from there. The islands are very nice but you should consider Bangkok and Chiang Mai as both have many great things to see. You should avoid Koh Phangan just because of the possibility of problems. Same for Phuket and Pattaya. Too many scams going there. Have a great time. You have selected the best time for weather considerations.