Well, make sure you've got your bulbs planted! Generally up to the end of December is dandy, but after that, it may be pushing it a bit
Also, consider putting out some food for the birds. Most seed heads and fruit have disappeared now, and all those tough little feathered ones who stick around these parts through the winter need a hand!
If you haven't done so yet, you'll want to rake out any leaves from trees that are prone to fungal leaf problems and dispose of them in the city green waste. Alternatively, other remaining leaves are good to leave in the garden to be broken down naturally and then turned into the soil in spring.
Flowers! Even though we're in the middle of winter, there are still plants making flowers. Often they are a bit inconspicuous, so you need to keep and eye out. Some of these include Hellebours (or winter rose) and the 'Dawn' viburnum (Viburnum x bodnatnese "Dawn")- it blooms off and on from November through February. Most other winter flowering plants don't really get underway until February- I shall post as I see them
For other winter interest in the garden, one can always turn to plants that produce berries or have attractive bark. For berries, other than the traditional holly, consider the beauty berry (Callicarpa)- it produces beautiful purple berries in fall that persist through winter. Skimmia is another great little shrub- the males produce clusters of red berries in fall that last through winter and also provide lovely flowers and buds in spring and summer. The hawthorn is another classic, though the birds often eat most of the berries, and what is left by this time of the year look a little worn out. Mountain ash is another tree for berries, they come red, various shades of pink, and orange. The birds quite like this one also. Pyracantha, a very thorny shrub, puts on a great display also, red and orange berries. As well, it makes a great place for the little birds to hang out and be protected. Snow berry is a native shrub that produces clusters of white berries. It's quite attractive, but the berries are poisonous.
As for bark, there are some great options also. The classic of course being the birch tree, either the Himalayan or European for that striking white exfoliating bark. Or the river birch, which has a brown exfoliating bark. The 'Autumnalis' cherry has a beautiful dark metallic red bark. The paperbark maple has a lovely cinnamon coloured exfoliating bark (this tree also has fantastic fall colour, and it doesn't get very big, making it a great candidate for the small garden) Both the red and yellow twig dogwoods are a good shrub for winter colour also- bright red and yellow stems add a splash of colour to any winter garden.
That's all I could think to ramble about right now... I'm sure I'll come up with more later