There is a ton of things you can work on during the winter. Groundwork and manners are huge things that will pay off if you keep your horse up on it over the winter. Short rides at a walk are great opportunities to work on bending, refining aids, pivoting, sidepassing, etc. Basically everything you would work at in the summer except at a walk.
I live in MN and winters are pretty miserable. I get in about 2 rides a week on each horse and a lot of times it's literally throw a bridle on and ride around the paddock for 15 minutes. It's done a lot of good for both of my horses and myself. That's when Soda perfected his pivots and Lily learned that any leg pressure doesn't mean RUN.
In my opinion it can almost be easier to work on those kinds of things in the winter because you aren't so focused on going jumping, running barrels, or getting out on that long trail ride. You're focused on refining both your signals/aids and your horse's response to those aids. You'd be surprised what you can get out of 15 mins when that's your only goal.