For your heritage site fix I would head to Siem Reap, Cambodia, and spend 2-4 days there exploring the temples. People say you can spend a week or longer there, but personally I think you'll be all templed out after a couple days. You can see the main temples in 2 days, most hostels/guest houses should be able to provide you with a map that has the usual routes outlined. So on one day you do one route, and on another you do another. It covers most of the main temples people come to see. Some people will tell you that biking around from temple to temple is cool, but I wouldn't do it. I ran into a Polish guy who tried to do that and he was exhausted and only got to see a couple of the temples.
You can rent a tuk tuk to take you to all the sites. IIRC I paid $5 for 12-14 hours of tuk-tukking. On the second day I paid $10 USD, because I wanted the tuk tuk driver to take me to the Landmine Museum, which is a bit out of the way... but well worth it. It's not a very happy place, but I highly recommend it. So anyway, the way I did it, is I followed those 2 recommended routes, and told the tuk tuk driver to stick to them, with some minor modifications. I had a list of temples I wanted to see, and some I didn't mind missing. The driver looked at my plan and suggested his own changes. Before we set off we had a plan we both agreed on, and then he spent the day driving me from temple to temple, and at one point to a restaurant for lunch.
One thing to keep in mind for your trip is that for instance your Thailand tourist visa will expire after 30 days if you arrive by plane.. and technically it's supposed to be 15 days if you arrive by land.. but when I entered by land coming over from Cambodia, they just gave me a 30 day visa anyway. So I dunno if that's standard or if they were just lazy. Anyway, overstaying your visit can be bad, so make sure you don't do it. In a lot of cases I bet nothing happens, but I would definitely plan your trip such that you never overstay your visa anywhere. On my trip I had everything planned out so that my flight home was 14-15 days after I crossed the border back into Thailand from Cambodia. So basically my flight home was supposed to be about 18 hours before my visa was due to expire. However like I said I ended up with a 30 day visa.. Either way though I would plan everything so that your visa doesn't expire anywhere. Look up each country's rules and stuff. For Cambodia you will need to bring 2 passport-style photos with you I believe. I even had to bribe a guy at the border because I didn't have any. So look into all visa rules and requirements for each country and make sure you're ready and that the timing works everywhere.
I loved the beaches and islands in southern Thailand, but they can get pretty touristy. One of my favourite parts of my trip was a kayaking trip by the western coast in Krabi province. But I also really enjoyed visiting some of the popular beaches. Some of them got pretty packed (Maya beach) but some of them weren't bad at all (Railay). There's a lot to see on the south-western coast, a lot of islands and beaches worth visiting. There's even a really old forest/jungle near there. I was going to go hiking there, but due to all the rain they got at the time some of the park was shut down so I never got to go. Either way, this is a really nice part of Thailand. Relaxing on some of those beaches makes you feel like you're in paradise. On the eastern coast is a party island that I would recommend, although it might not be your cup of tea. I went there last on my trip and went to a rave in a jungle, in the middle of the island. Danced all night to good house and techno. Good times and a great way to finish my trip. If you're interested, look up the full moon party island. I'm too lazy to look up the name
Oh, I recommend you take a cooking class somewhere in Thailand. A half day or day long one. I did one in Chiang Mai and it was very memorable. Plus I learned a lot and got a cool cookbook out of it. My favourite souvenir from the trip I think.
If you're in Bangkok and want a fun part of town to walk around, I had a brilliant time getting lost in Chinatown. It wasn't really that amazing I guess, but it was fun just getting lost in the side alleys, checking out the stores, the food, etc. Out of all the parts of Bangkok I visited, that was my favourite part of town to walk around in. Other than that you want to see the Grand Palace. It's beautiful. Floating market is neat.. Khao San Road is backpacker central. It's not must see but if you want to meet other backpackers, just kick back and have a beer, etc. that's not a bad place to do it. I met some people from Liverpool there just by chatting them up in a bar, and they gave me some tips on the rest of my stay in Bangkok. If you miss Khao San Road you're not missing much, but.. if you're in the area you might as well take a look. And if you "like to party" check out Soi Cowboy or Nana plaza. Walking through these places is.. interesting. Stay away from the establishments unless you know what you're getting into, though. I was just there to walk through the world's largest sex complex, since a week before that I walked through the world's largest religious complex in Cambodia. Yin/yang and all that. I also had a brilliant time at the Jim Thompson house & museum. I reckon it will be right up your alley, I'd look it up. Other than all that Bangkok has a couple other minor things to see, but I wouldn't spend more than 3-4 days in the city. The closest thing of note is probably Ayutthaya. I took a $2 train there and explored the ruins on.. foot I think. If you like ancient ruins and temples, and you're in Bangkok, you should probably head there.
Feel free to look through my trip report (linked in my sig) and ask any questions about any of the posts/photos you like.
edit: Oh and I almost forgot but read up on common scams. Be ready for them and know how to avoid them. Also read up on local customs, what not to do, what to do, etc. i.e. in Thailand if you drop money, DO NOT stand on it to prevent it from flying away, etc.
