Wow, the possibilties are almost endless.
Do an intro mountain bike ride (Friday eves) with PORC at Frick park?
Check out the schedule of Venture Outdoors activities. Try XC skiing, snowshoeing or ???
Take a hike in Allegheny Cemetery. They have a pamphlet that provides all you need for a great tour.
Look up the routing of some of the notable events from the past, and recreate one or part of one. Dirty Dozen? Fineview or South Side Step Treks?
Check out some of the local parks. They can be awesome in the winter. I like Deer Lakes (check the Amateur Astronomers of Pittsburgh schedule of star parties to coordinate a hike or a bike with an observatory stop) or Harrison Hills for the great view over the Allegheny River. Other parks offer other advantages.
Those are just thoughts of things I was thinking of doing this weekend.