The Minty Boost from Adafruit is a simple charger circuit that fits in an Altoids gum tin along with two AA batteries, and will charge almost anything (e.g., MP3 player, cell phone, camera…) that charges via a USB port. You could go the solar charger route for your cell phone, but the latest one of those is around $90, and only works with certain phones. Or you can get a rechargeable power pack from Energizer® (Energi To Go®) or Duracell® (PowerSource Mini), but these seem to run in the $40-$90 range. The Minty Boost is less expensive, and you get to build it! A kit (you will also need an Altoids gum tin) is available for $20.
On the other hand, I don’t think I’d want to try explaining this to the TSA people at the airport.