Mulch your garlic. Need a good source of organic straw? Call up Aurora Farms in Charlotte, VT. They grow grains for the Nitty Gritty Grains Company, and their bales of straw are just beautiful - no weed seeds, nice long strands with hollow stems to insulate from the cold, and best of all it is organic. You don't want to risk introducing persistent herbicide residues into the garden.
Plant bulbs before the ground totally freezes (quick!)
Plant some paperwhite bulbs for forcing. You can find out how right here.
Give your houseplants a trim, a feed, a little love. Check them for pests, wipe them off with a little neem oil if you see anything crawling around. This is a great time of year to give houseplants a little shower. The heat source in your house is likely quite dry, and most of your green friends like the feeling of rain. Recreate it for them. You can sing to them, too, while you are at it.
Make a few holiday gifts from the garden - herb salts, lavender sachets, cranberry apple chutney, herbal vinegars, calendula skin oil, or rosemary short bread. If you don't have any of the necessary garden ingredients, you can always purchase them and see it as inspiration for growing your own next year. Pinterest and the internet are brimming over with ideas and directions for making all kinds of beautiful holiday treats from the garden. The links above will get you started.

Make a garland, wreath or swag with stems, twigs, branches, seed pods, dried flowers, and other goodies from fields, woods and gardens. I love growing a hedge of Ilex verticulata and a small row of red osier dogwood for their berries and twigs make striking additions to winter greenery like balsam or pine or cedar.
Poke at the compost pile. Try to introduce some air into its nether parts so that it really heats up before the cold sets in. Add some fallen leaves, and a little manure from a horse farm or friends with chickens. You get the idea.
Look at seed catalogs! They are coming out now and the best therapy for December Darkness is those spring dreams of budding life and sowing seeds.