I took up the piano accordion at 45 but had an idea of how to read music. Lacking the accurate musical ear necessary to just "pick up" tunes I had to sit down and learn how to read music. It was hard at first but I've got the hang of things now, even if I am a bit slow. I can always learn tunes bar by bar.
Autoharp sounds good but you can learn the squeezebox (melodon, concertina or accordion) if you're prepared to spend five years getting the hang of it.
Try and borrow a lonely unused but sound instrument from someone near you to see if you get on with it. I did and I've never regretted it.