I've been thinking about a treadmill, too.
Why wouldn't a manual one work?
Alternative exercises/games for winter:
hurdles -- set up short, soft-impact hurdles (I used paper grocery sacks filled with newspapers for recycling, turned on their sides) and position yourself, with reward, on one end of the course. This works best with two folks, one on each end, but you can also work on teaching your dog to stay until called. Great if you have a long hallway.
hide and go seek. Hide yourself, with reward. Call. Rinse, lather, repeat. Be ready to laugh as your dog goes charging past your hiding place 2-3 times because you're not in plain sight. Also good for teaching dog to stay.