If you have access to a pool swimming is great too. Swimming strengthens your shoulder a lot through the entire rotation of the ball joint and can prevent all sorts of shoulder injuries from falls on the board simply by being strong and limber there. It's also great for the legs if you have any sort of knee problem already, as you can really work all the supporting muscles without putting any stress on the joint and ligaments. I started swimming again this fall and as I worked myself back up through the winter, I noticed big returns on the hill, especially cadiovascular.
It did absolutely nothing for my balance though.