For those willing to drive out closer to Detroit, the Henry Ford Museum is a great place to visit! Greenfield Village is worth it for that alone; there is no other place like it. Do recommend it as a day trip; there's a lot to see and do there!
Great trails and gardens to walk through. We had our third date here where Emily unfortunately got stung by a wasp on her toe. She persevered through the date (as Mike was cute) and the rest is history. Also near the University of Michigan's central campus.
For those with kids, this is a top-rated museum where we've heard a lot of great things about. Haven't been ourselves (need to find a kid to take as an excuse) but it's well rated amongst friends.
Want to go kayaking or canoeing down Huron River? Gallup Park is a great place to start. Also has a big play structure for kids nearby as well as paved trails to walk along the river. Mike and Emily love tubing down this river for a nice relaxing day.
An Ann Arbor classic; also near Kerrytown in Ann Arbor, which has some cool shops and restaurants nearby.
It might not be the Penn State Creamery, but Ann Arbor's got some good ice cream places too! Try this one in Ann Arbor; there's always a big line in front.