I used to do a lot of international travel in conjunction with the job I had then, and carried a Hassy kit similar to Jorge's - 50, 80, 110 (for macro) and 180, two A12s (color in one, B&W in the other), and assorted filters. It made for a heavy, but very versatile kit.

A reasonable tripod, or at minimum a monopod is good, but I've also found a chainopod to be handy in places that dislike tripods. You just drop the chain, step on it, and pull up gently for stability. Not as good as a monopod, but it hides in a pocket.