Banff National Park, since it's within my reach. The mountains are gorgeous, and there is just soooo much to do. Skiing of all kinds, hiking, swimming, running, along with the wildlife and nature and fresh air... We're in full tourist season now though, I recommend visiting at season edges.
How busy is Banff in the winter? Are there just as many tourists as in the summer? I'm a fan of snowshoeing and would love to spend a little time in Banff poking around.
Still a fair amount - not nearly as many as in the summer, there are more locals visiting the mountains for skiing and boarding. I would definitely recommend it! If you aren't REALLY attached to staying in the park, get a hotel or something in Canmore - it is literally just outside the borders and is way cheaper for that reason. Plus, it's right by Kananaskis Country, if you want to snowshoe around there.