Be sure you're getting enough light - are you suffering from SAD? Hmmm, some more ideas - take up sprouting - easy and inexpensive to start, nothing like growing any type food during the winter. Do you have access to a library? If so, check out tons and tons of gardening books to keep you feeling like you're outside. Get some houseplants. Increase the light in your house for a while and see if that lifts your mood. Make some easy birdhouses.