Small world - just got back.
We didn't get to Chiang Mai or the beaches. We stayed on the outskirts of Bangkok for the most part (gotta love it when family lives somewhere cool). Our trips out of the city were to Ayuthya - former capital of Thailand. A series of various temples, in different styles, and only about 2 hours north of Bangkok.
Also, we went northeast to Phnom Rung - very impressive temple on an inactive volcano.
In Bangkok, my faves were:
Chatachuk market. So huge, so much going on. We were there for about 5 hours, and I still didn't get a good idea of all the different things avalable. We'd see stuff and think "that's really neat." But you're left with the feeling - is the thing I want as a souvenir or gift? So many kinds of silks, tapestries, wood carvings - what are the things to get, what should you pass by? Easy to get swept up in a frenzy because things are relatively so cheap, but also easy to buy something that's very cool, only to find something you like much better just a few stalls or aisles away. I wish we'd had more time there. That doesn't even begin to describe the animal market that's part of it, or the food stalls that sell just about everything - yep, stir-fried bugs, etc.
Wat Pho - amazing temple - a bit overwhelming with how much stuff is there. Go early, especially if you want a Thai massage (this is the main temple where Thai massage is taught). You could spend a LONG time here, if temples are your kind of thing.
It's pretty much the height of the dry/hot season now, so bring your sunblock, ugly floppy hat, big bottle of water.
To get into some of the wats and other attractions (Wat Pho, Vimanmek/the Teak Palace, etc.) there are some restrictions on what you can wear...restriction on sleeveless items (tank tops), showing belly button, shorts or capri pants for women, showing toes or heels uncovered - it got complicated. There were variations in what was OK where - most places shorts/sandals/T-shirt was fine. Most attractions will lend or rent what they consider appropriate cover-up gear, but to avoid sometimes very long lines and the hassle of getting then returning the stuff, I'd recommend bringing along at least a button-up short-sleeve (to the elbow) shirt, a pair of tennis shoes or socks to wear under Teva-style sandals, and perhaps a very light-fabric pair of loose pants - comfortable, easy on and off.
We traveled most of the time with someone who speaks fluent Thai, but there were times we were on short outings without him, and I was really struck by the difference. If you haven't already, I'd HIGHLY recommend a language tape/CD and a bit of practice...just hello/thank you/please helped a ton (that's almost the extent of what I'd been able to teach myself with the tapes). There were so many times I'd wished I'd added things like how to say numbers, how to say a few simple sentences - would have made a world of difference. As far as numbers go, though, most people at the markets have calculators - I was able to bargain using their calculator to type my counteroffer.
Have a great time - I thought it was a fantastic place - but then if you've been there before, you already knew that.