I've just bought a banjo (yes ... I know the jokes) and I'm surprised how easy it is to get the basics right. Or at least to get an acceptable sound out of it. I wonder if a uke is a bit small to for older fingers, taking on board that you're talking about people requiring Care.

What about handbells or pop bottles? Certainly requires team work, and can be a real laff! (Re: handbells - I don't know much about them but my home church in s. Ontario has a set played by pensioners of the parish. Perhaps you could borrow some locally just for your elders to 'have a go'.)

sian