We love the Andaman Sea area and have done several 3 week trips there, one being for Songkran.

Our round trip normally starts and ends in Phuket for cheap flights and includes

Phuket-Phi Phi-Lanat-Krabi-Khao Lak-Phuket. I certainly wouldn't want to add in any more destinations as travel between places can be slow. You should try and spend a little longer each stay to really get the feel of the place.

Take a look at Bob and Janette's Travel Adventures for some ideas as they seem to travel that area regularly. Lots of pics of beachside places