I doubt I would walk between Greektown (Athenium) and Ford Field even in broad daylight. Take a cab! There is also a Marriott at the Renaissance Center and a Courtyard across the street from the ren cen.
Driving in Detroit during the day and rush hour can be pretty bad. Most of the other times, as long as there are no games or big events, it's pretty painless. I go down to something or other at least once a month.
If you have a car you can always stay in Windsor, Canada and drive over. Granted, you have to go through customs which can take 15 minutes or an hour (depending) but your money is worth %30 more and there are 3-4 decent hotels right downtown. It is also a much safer city. There is the Casino Windsor Hotel, Hilton and Journey's End Suites.