Anything athletic will help you physically get ready for next season.
I HIGHLY, HIGHLY, HIGHLY recommend yoga. I started practicing yoga last year beginning around early October. I would go to a class once or twice a week, and then run and stretch a couple of days too. By the time the first day of riding came around I was more than ready. I usually get mad quad burn and my back typically gets tight and sore the next day, but this never happened. Yoga is GREAT for riding - I know several pro riders (Jake Blauvelt) can't say enough about the benefits of yoga in relation to snowboarding.
Besides that, definitely leg and core development. I'm not a big weight lifter and so for me, this comes from playing soccer, calisthenics and slacklining.
Definitely pick up slacklining. It's cheap, fun, good for you and you can set up a line ANYWHERE. The learning curve in the beginning is a bit tough but once you get somewhat decent its an excellent activity for core and balance.