Even in January you can find something to do in your garden. On those mild days with sunshine you can get out and rake, spray your trees or even start pruning.
Days when that is not possible why not clean out the shed, and start to get those garden tools sharpened. You can clean blades with WD 40, getting any sap off the blades. Go ahead and start oiling all moving parts, cleaning any excess with a good cloth. Don't forget the wood handles on your tools, sand them down and then apply linseed oil. This will prolong their life for years. Make sure you get rid of your cloths properly, they can be a fire hazard.
During the dormant season take the time to remove any diseased or damaged branches from your trees, and do your corrective pruning. Pruning correctly is crucial to any tree health. Find good pruning instructions if you are doing it your self, improper pruning can result in killing the tree.
If you have fruit trees, it is also good during the dormant season to spray them. January is a great time for that. Spray a light horticultural oil, copper sulfur spray. Cherry trees need a bacterial canker using copper with spreader. Also check your peach leaf curl treatment with a spray of copper or sulfur plus spreader.
Raking up any winter debris is a good tip also. Some plants dropped leaves or flowering debris, and these can carry insect eggs or fungal disease. Have rose's, their leaves can carry fungal black spot. And for those Japanese camellias, they can easily spread petal blight. Raking can prevent any disease, so rake really good around tree base's. You can use your debris in a hot compost. Raking also minimizes disease and insect problems.
Chipping your leftover greenery can make a great mulch to spread around trees and shrubs, helping to protect their roots from temperature fluctuations. Also it helps to enrich your soil.
So start early when you can, and you will have more time in the spring to start planting those bulbs.