Thailand: know before you go
I did a good amount of prep for this trip and your to do list will vary. I was up to date on most of the vaccines I needed so I didn't have to do much on that front, but I did a good amount of planning with my packing.
Before your trip you will need to find a travel health clinic near you. CDC has a good website that can help you find one and give you an idea of which vaccines you'll need https://wwwnc.cdc.gov/travel.
You'll also need to ensure your passport is up to date and is not going to expire.
As with any international trip you want to make sure you bring some cash to exchange. I exchanged my money at the Atlanta airport. Exchange rates will generally improve the closer you get to your destination, but the exchange kiosks at the Chiang Mai airport close around 9pm so if you're getting in late you'll need to exchange some money before you get there. If you are taking a taxi you will go to the taxi counter in the airport, pay for your taxi, and take your receipt outside to where the taxis are parked. There are a lot of exchange kiosks around especially if you're visiting some of the larger cities.
Thailand is one of the places where you can NOT drink the water. There are plenty of convenience stores that sell large bottles of water, but don't drink the tap water and don't get ice in your drink unless you know it's made from filtered water. Most restaurants and hotels have filtered water and ice. Make sure you bring some Pepto Bismol pills just in case.