A kazzoo might be easy to do... A violin would be quite good as a real choice. You could have real hand poses made to look like fingering of notes and five or six different bowing poses and choose them randomly some way... A piano could be done too, you wouldn`t have to always be playing the exact notes of the tune as long as it looked quite convincing, 3 or 4 chord shapes would probably be enough. You could do most instruments so they look like they are being played properly but not playing the actual notes.