Another vote here for the harmonica!

Pianos are quite expensive and difficult to slip into your pocket. Guitars hurt the fingers of new players, causing them never to play again.

But with a harmonica tou've got a cheap instrument that's quite easy to play - you may well have a tune like Oh Susanna! off within minutes. And it doesn't stop there. You can learn to play cross-harp blues stuff, harmony/chord-type accompaniment, expressive solos. And all the time you're learning the harmonica, you're putting in time on the melodeon 'cos the notes work the same. And if you don't continue playing, you've only lost a few (insert local monetary unit here)s.