1. Campus Martius; it's Detroit's own Rockefeller Center. Ice skating at night with all the city lights, especially around Christmas.
2. A ski resort; there are quite a few in Michigan. I envision writing "will you marry me _____?" on the side of a mountain with food coloring (or something with color) and her seeing it as you are going up a ski lift. If that is not possible you can always ask nestled up next to a crackling fireplace.
3. In the driveway! On a snowy morning write "will you marry me?" on her car window. Be waiting for her at the door.
4. Sleigh ride; a horse drawn sleigh ride, it does not get any more romantic than that. This website http://www.sledriding.com/horsedrawnsleighrides.html lists 6 places in Michigan.
5. On a sled; you could ask as you are going down together on a 2 person sled! There are plenty of places to go sledding in Michigan, just find a hill. There is great toboggan run in Waterford called the Fridge (imagine proposing while going really fast down the run!)
6. Maackinac Island; it's open year round. There are places to ski and hike. You can surely find a romantic spot to pop the question.
7. Frankenmuth; you could propose on a carriage or any where. There are so many romantic little spots to discover.
8. Frozen shore of Lake Michigan; it is quite beautiful, especially by a light house. You could check out some places in Grand Haven or Ludington.
9. Joe Louis Arena; you can have a scoreboard marriage proposal for $100 or a live marriage proposal for $250 between the first and second periods of a Red Wings game. Added bonus: the money you spend on this proposal goes to the Children's Hospital in Michigan.
10. Coach Insignia; it's a restaurant on the 71st and 72nd floors of the Rencen in Detroit. It has a panoramic view and so be sure to reserve a window table. It is the 2nd highest restaurant in the US.
So... I couldn't just pick 10 :)
11. Detroit Institute of Arts; this is perfect for art lovers. Choose your or your soon to be fiancé's favorite piece of art to propose in front of.