If you can already play pennywhistle, try tabor pipe. It's made for only one hand - it only has 3 holes, but takes some skilled breath control. With a bowed psaltry, you only have to hold the instrument with your left hand while bowing with your right. Mountain (or lap) dulcimer takes two hands, but one style of playing involves only fretting one string with a piece of wood (or one finger). So your left hand would only have to hang onto the wood and move it up and down the neck. Added bonus: they're extremely simple to learn and not too expensive.