I'm lucky to be able to ride year-round (I'm not gloating, really). But, some of my clients live in snowy climates or just like to go to the snow from time to time. One of the best cross-training activities for cycling is snow shoeing. You can walk, you can run, you can play games and have contests. The cardio is great and the muscular is very complementary to the cycling action. The folks who are really into it are pretty hard core.