I agree with darknj, 18-135mm. Here's my several reasons : - how often you change the lens in your trip? Imagine there's split seconds interesting composition and you have to change the lens first before taking picture. - do you really care about sharpness and really want to print your images (in a large size)? yes for pixel peepers, but for your own documentary, taking not sharp picture is better than not taking picture at all. - are you a prime shooter and really know how to compose with each focal length you have? if yes, you can reject my first and second opinion and please take 16,35, 56/90. Tripod is a must for me, choose a lightweight one. Flash is depend on you. Last, have a good trip to Thailand!