I use the Orange Whip a lot during the winter. Swing it in the driveway even if I have to bundle up. Keeps me stretched out.

I picked up "The Putting Thing" from WhyGolf this summer....and it's really helped with short putts. Same concept as using tees/balls/etc to create some "gates" for the putter head and/or the ball to check your start line, but it's compact, portable, super fast to set up, and has some alignment aids to help with eye position.

The crazy thing is how fast I can lose my stroke after a few weeks off. At the end of the summer I was pretty good at rolling the ball through the narrowest gate, after a few weeks off I could barely get it through the lower/wider gate settings consistently. Just one more thing to add to the winter grind.