If you don't want lots of holes in the walls you could fix a solid wood batten along a wall, with just a couple of firm fixings (or even supported by posts)., and hang the instruments from hooks or whatever on it.

I'm all for keeping string instruments on walls - they look attractive, and they are readily to hand when you want to play them, and cases have a kind of imprisoned quality. "Only a bird in a gilded cage"... But then I don't live in an earthquake zone (touch wood).