Button Box Concertina - One long weekend and you can play a dozen songs by ear....cost about 150.00 US in a pawn-shop.

Unable, to transport a piano I "latched onto" any keyboard instrument I could "borrow", mostly organs, as I traveled, eventually progressed to fullsized accordian (mailed it ahead and also lugged it around by train - too bulky still.

Extensive backpacking, (all three US ranges) led me to, recorder and harmonica....size and weight were primary concerns.