Autoharps, though fairly compact, are heavy and sound like ratshit if they're even slightly out of tune. By the time you've walked half a mile I garantee it will have been jiggled way out of tune. The mandolin, although REALLY compact, needs a lot of technique to be an accompaniement instrument in other than a very few keys..likewise the concertina. As far as I'm concerned there's only one instrument that fits the bill in all ways..the baritone uke, or tenor guitar.(they're VERY similar) Both are dead simple to play (if tuned like the 4 high strings on a guitar) and with a capo, can be hugely versatile. They're not that difficult to find, are usually cheap, can (easily) fit in the above luggage container on a plane, and if you want power....can be equipped with a pickup. Mine is a Regal "courting guitar" from early in the century and cost 75 bucks (Canadian).

Rick