Advil, Aspirin, something for an upset stomach: Different countries, different food. Keep in mind that your stomach might not agree with something that’s a regular (and even quite likely delicious) ingredient in Thai dishes. So bring something to calm down your stomach. Also, make sure to pack Advil, Aspirin or whatever equivalent your country of residence offers just in case. (Do check flight regulations of your airline in case there are restrictions about bringing medication on board.
Do not forget your favourite sun screen and bug repellent.