Read a book that your eyes haven't seen since pilot school, network with other pilots, go "split" some time with a pilot friend and add to your Actual Instrument and Cross Country time or go to a free aviation seminar. Find some interesting seminar topics on the AOPA site or the FAA site, find their Events List here. Most recently they presented a seminar titled "iPad in the cockpit". By the way if you haven't subscribed to Fore Flight, do yourself a favor and look into this super helpful ap.
Career Magazine
There's usually not as much flying for us Jump Pilots this time of year in the Northern Hemisphere, reference the snowy photo below. But there is plenty of other aviation related things that you can do to keep your head in the game.