First up is the Michigan Military Technical & Historical Society. They have 2 available offers: $12 for two admissions with a $15 gift shop voucher and $15 for family admission (two adults and unlimited children) with a $25 gift shop voucher. The museum it's self has an interesting assortment of military history to see. While you're there, you can look into their WWII roundtable events, re-enactor workshops, and the Young Modelers Club.
If military history isn't your cup of tea, then look into the Flint Institute of Arts Groupon which is for yearly memberships. $25 for a family membership and $50 for a membership with reciprocal benefits. A year of fine art for the family is well worth $25. It's a chance to explore the world and it's cultures without leaving Michigan.
Perhaps you'd like an art museum closer to home, there's a Groupon for that too. This one is for the Museum of Contemporary Art Detroit (MOCAD) memberships. With 3 options, there's an option for any situation: $30 for an individual membership, $45 for a dual membership, and $49 for a family membership. This membership give you access plenty of perks, but the best has to be the discount at local businesses. It's easy to make a day of the newest exhibit and supporting the local economy.
Make sure to check out these Groupon deals before the end!