That time of year I would head for the Phuket or Krabi area. In Phuket you might want to consider Kata or Karon if you want a livlier resort. Patong is the livliest but has a seedier side as well.

Krabi...Ao Nang is the livliest resort and a useful staging post for trips to various beaches.

There's plenty to be down from both areas - rock climbing in Krabi, sea kayaking, elephant treks, temples to visit, national parks within striking range. Koh Lanta might be worth a look as well. Its quite possible to combine all these destinations if you are prepared to resort hop.

when you decide where you want to go - trying pricing it up on your own and then go to a travel agent for a quote. We have found that there is not one answer as to whether to go DIY or not. Best to explore all options. Start with investigating the cheapest flights to Bangkok for your dates.
Whichever hotel you choose in Bangkok its important for it to be near to the Skytrain as this is the best way to move around the city. Saying that, we have stayed beside the river at the Bangkok Marriott Resort and Spa and the hotel boat will take you over the rivet to the Skytrain stop.

The Skytrain is the quickest way to move around the city, it is however fairly limited in its coverage - there are many places it just doesn't go. Good fun though.
I would definitely recommend Ao Nang as a base - not the best beach, but many stunning ones, including Hat Rai Lay are a short longtail boat ride away. One of my favourites is Hat Nopparat Thara - very quiet even in the peak season. There are two parts to this beach, the best part is over the river by the national park.
There's plenty of non beach activities in Krabi too. Try the Tiger Cave temple complex and climb 1200 very steep steps to the top of a limestone cliff to see the Buddha footprint. Very tiring but worth it for the view.
The first time we went to Thailand, we booked with Kuoni, who I can't recommend highly enough - they were great. If you book any tours with them, they will be through a local company called Tour East. Excellent, knowledgable guides (Which are handy in some of the Wat). They do always seem to drop you off at one store or another at the end of the day, tyring to sell you jade or so on. They do take you back to your hotel eventually.
Since then, we have booked independant trips, which I prefer. There is so much choice and flexibility that you won't find in a brochure. I would probably have saved around £300 pound if I had booked my first trip independantly.
have a look at AirAsia,com fot your internal flights. If you book early you will get a return from Bkk to Krabi for about £20 each including taxes.