One of the nicest discoveries I made when I got my first mandolin was the way it could be played, pleasantly and quietly, without taking up extra space, in the passenger seat of a car (or other similar places where a guitar is too big, a fiddle is too awkward, a pennywhistle is too loud, etc.) So the mandolin is my favorite instrument to bring places these days ...

I haven't had trouble bringing a fiddle (actually a viola) on planes as a carryon -- except one time as a kid, when my mother made an unfortunate joke at the security gate about machine guns in violin cases, but that's another story -- but I haven't tried with bigger instruments. I know someone who plays cello and says he has to pay for a second seat for the instrument.

JZG