Look out your window. We've got some snow. It's the holidays so you really have no excuse not to go sledding. With that in mind, I am posting some links to some classic articles about sledding in Ann Arbor:
Guide to Ann Arbor: Sled-DANG! In A2 from Damn Arbor's very own Quinn DavisI will add this: Huron Hills is probably my favorite sledding spot in the city (see this article on the Chronicle). The Arb is pretty sweet for sledding too. But the frozen hummocks of grass kind of hurt your bum as your sled there. And it's borderline illegal. But really it doesn't matter where you sled, as long as you get out and enjoy yourself.
Sledding on Arborwiki
Guide to the Ann Arbor area's best sledding hills by Ed Vielmetti