1. A durable, lightweight, smooth-rolling wheelie bag that holds a lot and doesn't tip over easily. 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. Means of communication. Check all your devices (phones/tablets) and chargers. To make sure you have everything in order, just get an organizer for chargers. Make sure you don't forget your Pond Mobile sim-card to stay connected with unrestricted high speed internet access and no hidden roaming charges anywhere.
3. 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.
4. One-quart Ziploc bags for getting liquids, gels and toiletries through airport security — pack a few extra ones in case one rips or someone you're traveling with forgets to bring one.