3 1/2 choices:

1. Rental car. 'Nuff said. Parking very available downtown (albeit at a cost)
2. Metro Cabs at the airport (cabs)
3. Metro Cars at the airport (airport limos)
1/2 SMART bus from DTW to downtown (takes 1 1/2 hours, very much a meander around downriver, way impractical for what you've got in mind...only practical on mid-day journeys during the week)

Only comment is that if you're very late (past about 12:30 am) there have been reports of no cabs, no dispatchers, no anything. issue is that the female dispatcher does not have a ladies' restroom at the location (wherever it is) and has to go next door (!) to the fire house to go to the bathroom (!)....was just this week on the Freep blog otherwise I wouldn't have believed it.