Have you checked out the Pure Michigan travel and tourism website?
http://www.michigan.org/

Michigan, including Detroit, has so much to offer!!! You are bound to find something to enjoy!

I suggest that you consider Michigan's thumb....Port Huron has beautiful views of the water, a couple really nice museums; further up M-25 you can stop at Lakeport State Park for the day and then keep going through a number of small towns such as Lexington and Port Sanilac and Harbor Beach, all with history and quaint small town atmosphere. Nice B&Bs in the area too. At the tip of the thumb, enjoy Port Austin and then down to Caseville and Pointe aux Barques. Then head inland over to Saginaw...the Marshall Fredericks Sculpture Museum is at the University there...and downtown is a lovely historical museum. Back down I-75, enjoy a day in Frankenmuth, an old German village with lots of shops and restaurants. Shop at the outlets at nearby Birch Run. Then back down to I-75. Before hitting the Ohio border, consider a visit to Ann Arbor...there's a museum of natural history at the U of M, plus a Hands On children's museum, and lots of wonderful restaurants on Main St.

So much to see and do!! Enjoy your time in Michigan!!