With kids Greenfield Village is fun, but with the hot weather, Great Lakes Crossing is cooler.
If you do go Zoo, head to Toledo, as they have more shade then Detroit.
The State's link: Travelers Landing Page - Pure Michigan Travel is a great resource for one day trips. If you know a little in advance on the places you will be going, you can contact the Chamber of Commerce. Many Chambers can provide you with coupons for restaurants and such - http://www.michigan.org/default.aspx
There is also a really inexpensive indoor playland in Howell called Castway Cafe. The kids can play all day for between $3 and $6 and you do not pay for kids over 13 y/o. Their food menu is pretty decently priced. Castaway Cafe- An Exciting Place to Eat and Play - http://www.castawayplay.com/
Ann Arbor Hands On Museum: Hands-On Science Museum for children of all ages | Ann Arbor Hands-On Museum - http://www.aahom.org/
There are a bunch of indoor waterparks in Michigan and Ohio. Here is a link: http://themeparks.ab...indoorwater.htm