As a writer, I can make any excuse to spend weeks without leaving my house if I'm on a deadline, which means it can be a long, depressing time in solitude if I don't watch it. I've found leaving the apartment, even for an hour or two, helps. I now work outside my apartment among the living with services like Spacious.
There is no better time to skip town than when it's dark, cold, and lonely where you are. There are plenty of airline deals after the New Year, and there are countless tickets to destinations for next to nothing. If you're so poor you can barely pay attention, drive or take transit to visit a nearby beach or town. Last winter, I took the subway to various places I'd never seen before. A change of scenery really helps.
Last year, I signed up for a photography class and a business course, which meant I had to leave the house at least twice a week. This year, I'm going to join a hiking group and I'm taking a knitting class. Find something you've always wanted to try and make this the season you try it. It motivates you to learn and it helps you meet people you might otherwise never know.
As an introvert, I should throw this in here: I am positively drained by long-term contact with lots of people. I have to be alone for a while just to feel normal again. That said, there's a point where I know I feel better and I'm just using "introvert" as a cover for depression. I don't have to talk to anyone, but just a walk to the corner coffee shop seems to help.
I try to anticipate feeling less motivated through the winter months, so I plan at least one outing every week with a friend or with family. I usually ask friends to meet me at an exercise class before brunch so I can call it multitasking, but sometimes it's a movie night at a friend's place or mine. Bonus round: going out dancing with friends or family counts as time outside the house, staying warm AND exercise.