The mountain bike trails around here are utterly empty during the week. I'd be lucky to see 2 people in 2 hours. Great trails for the dog to run and a reservoir for her jump in.
Can't do it on the weekend, too many people.
Usually though, my wife takes her running around the same place and when she gets back, I go mountain biking. The pooch likes running with my wife more than following a mountain bike, it's slower and she can sniff and explore more.
I'd you're not in a crowd, it's great.
Backcountry skiing gets a little tricky if it involves deep powder.