I've got to go with Catspaw on this one. I play several stringed things. But for travel, I always carry my Appalachian dulcimer. I take mine with me on business trips to while away the time in hotels, as well on camping and floating trips in the Ozarks. It's light and compact enough that it fits well in the airlines' overhead bins or under my seat.
Another advantage to the dulcimer is that it's easy to learn to play, while affording a picker the opportunity to get as complicated as he/she wants with chording and finger picking patterns. And best of all, you can tune it to play in the keys in which you most comfortably sing.
What more could a person want in a travel instrument?