There is a parking garage adjacent to the Marriott Renaissance Center (not operated by the hotel) which might be cheaper (honestly don't know the pricing...I work in the Ren Cen so parking is never an issue for me down here). Non-valet parking isn't so easy at the other hotels (but it is available)

Really the only place where you can do without a car is staying downtown (or in Birmingham, MI...leafy wealthy suburbia with good and varied shopping and hotels ranging from Holiday Inn Express to Townsend Hotel).

The People Mover does a big loop encompassing much of what you'd be able/comfortable walking to in downtown (Fox Theatre/stadiums are just beyond it along Woodward.

The Woodward Corridor is the core of the city and the interesting areas are reasonably safe. It gets sketchy, then, for about a 5 mile stretch (buses are safe, though), until you get to Ferndale (inner ring suburb...downtown at 9 mile and Woodward), then Royal Oak, then Berkley, then Birmingham as mentioned above.

If you want to take the bus to the suburbs, you need to take a SMART (orange/suburban) bus not a DDOT (green/city) bus. There are some rules about where SMART can pickup and let you off...just ask the driver.