Play a hammered dulcimer! Busker's dream instrument. It helps if you've got some tunes worked out, but because it's both resonant and has a limited range of keys, you can get away with doing barely more than hitting the strings and it will still sound nice. People generally haven't got a clue what it is, so it attracts attention - especially from children, and as soon as you've got their interest, their parents' wallets fall open!