Ktrax lets you ski your bike
How's this for an all-terrain workout device? A bike that can be converted into skis.
The Ktrax Rear-DriveKit is a universal attachment that replaces your rear wheel with a track drive system, so that you drive on previously “undriveable” surfaces, like snow and sand.
If you find yourself in the snow, you can also exchange the front wheel with a ski. It isn't designed for deep snow, but rather groomed or packed snow.
Ktrax has designed the adaptors for both adult bikes and kids bikes, so you should find one to fit your bike.
Available some time this year, the adult rear drive kit costs around $400, while the kids version is $200. In addition, the ski kit costs another $140, while the kids costs around $90.