Stand up paddle boarding is a great core, balance and aerobic work out. It also gets you on the water when there isn't enough wind to kite.
I would love to try this, give me something to do when nobody will pull me behind the boat wakeboarding! Too bad they are so pricy though... maybe John Lock can build one for me!
Personally I longboard and skateboard in the summer, it might not directly hone your snowboarding skills but it keeps me sideways and having fun. Wakeboarding as well when I can get to the lake but its definately not a daily event.
Sounds like mountain biking would be a good choice if your lacking pavement, been a while since I was into that but its super fun and great for conditioning and excitement.