Beaches in such areas as Krabi, Phuket, and Koh Samui are reached by internal flight - some of these can be a reasonable price as they have the no frills airlines in this part of the world as well. The resorts of Hua Hin and Pattaya are reached by road - a two to three hour journey in either case. Hua Hin is a quieter resort, Pattaya very lively with something of a notorious reputation. The beaches you see in the brochures tend to be the in the areas you have to fly to. At the time of the year you are going Koh Samui might be best weatherwise.

If your going that far its worth having a couple of nights in Bangkok as its a great city with loads to do.