Depending on where I've lived, I've been into hiking, backpacking, mountain biking, rock climbing, fishing, and kayaking. What's fun for me might not be for you. Maybe pick something that will keep your legs and core strong and improve your balance during the off-season. I'd say mountain biking or climbing would be good bets, but both can be expensive. Go hike up something steep in the meantime. Decent hiking boots can be gotten fairly cheaply, and you'll get to see some neat stuff. If you do backcountry riding at all, hike the same places you ride.