Zingerman's Roadhouse! You get great food at an Ann Arbor institution. I highly recommend the fried chicken. You can always take in the Deli, but that's more a lunch time thing than your first night in town.
The restaurants downtown are mostly ok but not inspiring. The Roadhouse is a little expensive, but your food and your experience will be worth the extra dollars. Zingerman's waitstaff really know their food, and they'll make sure you're happy.