@yahunatan If you are just looking to build some muscle, but aren't looking to go full "body builder" (climbing by itself doesn't really get you there), indoor climbing can be a good alternative to traditional gym fitness. It can be a great full body workout.
Beginners mostly use their arms to pull themselves up, but you'll quickly find you also have to use your legs and feet a lot and then engage the core for a lot of moves too.
The variation keeps it fresh and fun. I'm particularly fond of bouldering; short, challenging routes that don't require a rope.
It's also now an Olympic sport that will be in the Olympics for the first time this summer.
Could be interesting if you're want to try something a little different or mix things up a bit in the future.