I haven't done it since they revamped it last year, but the rouge tour is pretty damn cool even though it is a bit on the expensive side. The Detroit Zoo is also a good call, they just opened their new world class penguinarium and it's AWESOME!
Closer to Downtown there's the DIA if you're into that kind of thing and Belle Isle is gorgeous now that the State took control and made it a state park, though it's kinda sectioned off a bit right now as they prepare for the Grand Prix next weekend.