A must when you are in Detroit is the Henry Ford Museum and especially the Rouge Factory Tour. As a big car fan, you definitely have to swing by here. Another big highlight is the Greenfield Village, located right next to the Henry Ford Museum. For all the attractions here you need at least two days to fully enjoy them. The good news is that your ticket is valid until you use it for one of the attractions. If you buy the main attractions (Museum, Factory Tour, and Greenfield Village) you can visit the museum and factory in one day and scan your ticket and visit Greenfield Village the next day.
I recommend you the following itinerary.
Some advice for your visit.
I recommend you to take the factory tour fist and then do whichever other experience you booked. This gives you the chance to spend as much time as you want in the factory and after that, you can plan freely and visit the museum or village. Everything closes at 5 p.m. but the last bus for the factory tour leaves at 3 p.m. Buses run every 20 minutes to the factory and back. You can hop on any bus you want to or from the factory. There is no limit how much time you can spend where.
If you can visit during the week, means fewer people. If you can visit on a Monday or Tuesday because during the school year there are most of the time field trips to this places from local schools and then it gets crowded and sometimes loud. Depending on the age of the kids ;).
Besides the above-mentioned attractions, you can buy an add-on for the village which allows you to ride all the rides inside the village. This includes the Model T, steam train, and horse carriage.
Another feature is the Giant Screen Experience. You can watch different documentaries in the on-site movie theater.