Thailand!
I loved Railay (sometimes spelled Rai Leh) in Krabi Province -- gorgeous, very small, cheap, and stocked with fun stuff to do that isn't as much of a tourist gauntlet. Not that Thailand has a problem being laid back, but this place was the most laid back of all the beaches I hit, and I found amazing food everywhere (dining's a mixed bag on the coast). You can take a boat there from Ao Nang or Krabi, then double back and head to Phi Phi or or whatnot if you're beach-binging.
Phi Phi is breathtaking if you face the lagoon for the entirety of your stay.
Ko Chang is also beautiful, it has a gnarly jungle interior and peaceful beaches, the pace is similar to Railay's, and it's close to the Cambodian border. Transportation around the island was an issue, it's a few km between each town and scooter rentals (death wish) or sorngthaew (pricey) are your only options.
But! The buses between Ko Chang and Bangkok are legitimate, comfortable and friendly affairs, in stark contrast to the ones headed toward Surat Thani -- please, if you value your time, money, safety and sanity, take the train south!
For a non-beach recommendation, try the Forensic Medicine and Parasite Museum at Siriraj Hospital in Bangkok, just across the canal from Banglamphu. It's like an Asian Mutter Museum, full of exciting, gruesome crime artifacts, loads of dioramas depicting various tropical maladies, and a cool exhibit on recovery of victims after the 04 tsunami. The handful of English descriptions are charmingly blunt; rent the headphones.
Also everything in Chiang Mai is wonderful. Spend as much time there as you can. Go to Sweet Love Coffee at every opportunity. It's right outside the walled city on Thanon Tipanet. It has an outrageously cute yet sleek/practical aesthetic, is staffed entirely by anime characters, and has free wifi and a fancy imac for you to use. I was practically in tears when I came up north and drank decent espresso again, and after you've used a few internet cafes in Thailand you will appreciate the graciousness of their free wifi and computer, especially because there are next to zero people here hogging bandwidth due to the location (tourists tend to stick to the eastern walled city and students are out west, but you should already be renting bikes or scooters here anyway, so it's a five minute ride from anywhere).
That's probably more than you need. Have a great trip!