Haroun K.
Yelp
The arb is my favorite place in Ann Arbor, just outranking the Botanical Gardens. Surprisingly few non-undergrads seem to know about it, though. It is the closest thing to Central Park you'll find around these parts, with walking/jogging trails, trees for studying, flowers for admiring, and big, big fields perfect for running around or reenacting a Civil War Battle (actually, no, please don't do this). And there is the Huron River, where the ducks are as fearless as the squirrels in this crazy city, and where locals will bring their dogs for a quick swim. I don't think I'd go in the water, but that's just me. The dogs really do love it here, though. If you are planning to go to the river, bring some sturdy shoes, as it's a steep descent.
You can park in the Blue Lot behind the Markley Dorm (on Washington Heights), provided it's after 5 pm. There is also some metered parallel parking right in front of Markley, and meters run till 6, I believe. It is way easier to park in summer, when the students are away. There is another entrance off of Geddes, and you can park for free on the side streets there (but good luck finding a space!). Entry to the arb itself is free, of course.
I did see one portapotty by the peony garden, and I think there's a bathroom at the information center by the Washington Heights entrance (open weekdays during businss hours), so there's no excuse to defile the river!