I have to agree with everyone about Zingermans, they have good stuff. If you're looking for something more upscale, Aventura is our new favorite haunt. As for what to see (I'm assuming you're not familiar with the campus at all):
1. The Diag is the big open space in the middle of campus. There's a bunch of diagonal sidewalks through it, hence the name. There's some really old Elms there (which is only cool if you're a tree nut), the University did a lot of work to protect them from Dutch Elm Disease. There's a big metal 'M' plaque in the center of the Diag. Don't step on it. It's bad luck to do so before completing your first exams.
2. The Law Quad is just beautiful. As is the old above-ground portion of the library. It's like something straight out of Ccambridge. I had my engagement photos taken there.
3. I encourage you to wander through some of the class buildings, and definitely the libraries. You can really get lost in the grad library.
4. It's small, but the Natural History Museum is nice. There's dinosaur bones, and a lot of stuffed "Michigan Native" animals. The lion statues out front also have legends attached to them.
5. The student Union is worth a look to see where and how students really spend their time. You want to go into the basement, that's where people really go.
6. Next to the Union in a little square is a giant cube (it's called The Cube, very original). It spins. The story is that the president has to spin it every morning to start the school.
7. You probably can't go inside, but it's worth it to do a drive-by of the stadium. You can't really get an idea of its size from the outside, but it holds a LOT of people. Every parking lot around it, every golf course, every driveway for miles, will be filled with cars on game days.
8. In the downtown area you never know what you'll see. The stores are quirky. There's this gorilla that plays the violin (though probably not in February). If you look close, you'll find "fairy doors" hidden all over the place.
I could go on. I gave the best 8 years of my life to that town. I, my husband, my parents, his parents, and one of my SIL's are all alumni. So, I'm partial, of course, but you'd seriously be hard pressed to find a better all around school for your DS. Some schools have better academics, and some have better social/school life, but none have a mix as good as Michigan's. Ann Arbor is a great town.