I've been there several times now, and I really love the Thai Chili Sauce. I think that would be a great addition to your kitchen. It can go on things such as sweet and sour pork, chicken or fish, use as a stand alone sauce, or barbeque little chicken wings in it. We had this done for us while we were there. It has just enough kick to it. Not too hot, but just enough for some great flavor.

I second the chili sauce, and you can probably find some great spices and curry mixes for cheap! I've been there twice, and I didn't really have a chance to cook for myself either time, but I ate some incredible food there.

Sangsom!(technically not a kitchen item, more of a bar item, but I think we all could use a bottle!) I brought back so many kitchen goodies, mostly spices, but I also loved the oyster sauce I got in Thailand as well as the chili peppers. I also bought a cutting board from a kitchen shop that I still love. Just have fun!
p.s.I am a bit jealous...

The steamer baskets to make sticky rice are beautiful if you appreciate hand-crafts and you happen upon someone making them.

Also there is a peeler designed to make the long skinny shreds of green papaya for the salad, that is nice too- does not take up too much space in your luggage.

Not sure if it's allowed through customs, but Golden Boy Fish Sauce is a favorite of mine.

I wouldnt waste my time with fish sauce, curry pastes, and chilli sauces. Those can be found just about everywhere these days (depending on where you live of course). Instead, try to find some snack foods and drinks to bring back that you cant find here. But ya, not too sure about customs though...

and maybe some good local spices like nutmeg, whole green peppercorn, and curry powder.

1. Read the US Customs guidelines:
https://help.cbp.gov/app/answers/detail/a_id/82/~/travelers-bringing-food-into-the-u.s.-for-personal-use
2.
Exotic dried fruits, such as durian or longan
Spices such as canned green peppercorns
See http://grocerythai.com/ for a look at some common groceries.

Curry pastes (green, red, panang), papaya salad dressing, pad thai paste. All can be found in the markets and are vacuum sealed to maintain freshness. Soooo good!

Again, perhaps don't bother with chili sauce and the like. Asian markets here carry all that stuff. For example, I have a bottle of Golden Boy fish sauce in my refrigerator now, and I only had to go five miles away to get it. Now, if you happen to fall in love with a really awesome brand that you've never seen here, then go for it.

I second the spices. Snacks or candies are also a good bet, no matter where you'll be travelling. Maybe really good tea or coffee, but these may be verboten by customs.

I went a few years ago and still regret not picking up one of their cutting boards. Crazy thick slices of some sort of hardwood. In my experience, most things thai can be found in large cities. Worry about taking back lots of pictures and great experiences. (Also if you take a petpto tablet every morning you can eat pretty much anything)

Seeds to grow Thai herbs and vegetables for cooking.

Honestly - you can get the foodstuffs and cooking tools from a variety of importers. I brought back some awesome servicing pieces, lacquerware and silk textiles on my trip to Thailand/Burma/China. Find something interesting you can use for years. You'll thank yourself each time you pull it out of the cabinet.

(that would be 'serving' pieces...)

maybe Keffir lime leaves. They can be difficult to find here, they can be frozen and they weigh nothing to transport.

And maybe some cool serving or cooking implements. I wish I had gotten a basket for making sticky rice. 3 yrs later after our Thai honeymoon and we're still obsessed with mango sticky rice

Thanks everyone! Will be listing every recommendation given. Can't wait for the trip. Cheers!