I don’t know if anything could ever totally replace skateboarding for me (which is a tough reality), but rock climbing/bouldering is very comparable to me. Problem solving, lots of different techniques to learn, physically demanding, and often requires lots of attempts on the same thing until you finally get it. And there are different levels of “getting it” in terms of style, technique, cleanliness. And if you’re into it you can also take it outdoors vs the gym (street vs park, etc). It also works different parts of your body in case it’s a leg situation you’re dealing with re: not being able to skate.