One of the best things you can do for your snowboarding just may be to talk a walk in a park. The Twin Cities are filled with parks and other parcels of land where you can do a little "extreme" walking. There are many to choose from, but Battle Creek Regional Park, in Ramsey County, is a good place to start. So is Lebanon Hills Regional Park, in Dakota County.
Granted, hiking isn't going to replace snowboarding, but it can be a good supplement. It can help out in a time crunch, for example. Perhaps you don't have enough time to put everything in the car, drive to a ski area, gear up, ride, and then do everything in reverse. Chances are, though, you'll find a good hiking location that's closer than a ski area.
Winter hiking can also, like snowboarding, be a physical test. On the one hand, it can be "just walking," especially if you go on dry, flat ground. But try hiking up and down hills of crusty snow or ice, and the challenge level goes up.
Winter hiking also has some of the benefits of snowboarding--minus, of course, the p-tex base and thrill of speed. After all, one benefit of snowboarding is being outside, rather than consigning yourself to the couch an getting cabin fever. You get the same benefit by going for a walk in the park.
It's also exercise, which helps your snowboarding. This is especially true if your hikes take you to hilly terrain.
If you do winter hiking where you normally walk or ride a bike in the summer, you'll also get a new appreciation of how the snow transforms the landscape. Unless you hit golf balls at Hyland Hills' driving range, or ride the mountain bike trails at Afton Alps or Buck Hill, you will not have that experience at a Twin Cities ski area.
Finally, you can also use your quiet times in the woods to play mountain architect. As you hike up in the woods, look around. If you had the power and ability to place a lift, where would you put it? Where would you cut a few trails?