1. A durable, lightweight, smooth-rolling wheelie bag that holds a lot and doesn't tip over easily. REI makes the Stratocruiser 25", which comes with handy backpack straps and a detachable day pack. Four-wheeled bags are even less likely to tip over, but they're slower to walk with and prone to rolling away when you're standing in line somewhere.
2. A slim digital camera that turns on and focuses quickly. Canon's PowerShot series does both jobs well.
3. Duct tape...because you'll inevitably wish you had some. Packing a whole roll is annoying, so take a 2-ft. strip and wrap it around a pencil, a straw or marker.
4. A copy of your passport. You'll probably never need it, but if you lose your passport, having a copy greatly expedites the process of getting a new one. A scanned copy that you e-mail to yourself works fine.
5. One-quart zip bags for getting liquids, gels and toiletries through airport security. I always pack a few extra ones in case one rips or someone you're traveling with forgets to bring one.