Charger block and charge/sync cables
When traveling, bring one charge/sync cable for each device -- you never know when you need to charge more than one simultaneously. Retractable cables can save space, but they tend to break easily, so using rubber bands to corral more-durable cables is a better idea.
Also bring a charger block that can power two devices -- most do just one device, so you need more outlets and chargers. And be sure it supports tablets' higher wattage requirements on at least one port. If you rent cars a lot, get a second charger block that plugs into a cigarette lighter port. I like PowerGen's dual-port USB wall charger and its car charger due to their compact size, nice design, and reasonable prices.