Start the day at the Münsterplatz market (best on Saturdays). Try the fresh-pressed carrot-apple-orange-ginger juice and some tofu wurst. If you're the academic type, you might also be interested in walking around the university campus to see the statues of Aristotle and Homer, and the many bike racks.
Take the S-bahn to Vauban, a "sustainable model district" that is the epitome of Freiburg's ecoconsciousness. Upon arrival, you might want lunch before heading into the Black Forest. Süden, part of the bunker that Vauban has converted into a community center, offers everything from Schnitzel to veggie lasagna and satisfying salads.
Ready to hike? I wish I could come up with a way to link to a map, but basically you want to walk through the neighborhood of Merzhausen, uphill, and into the Black Forest. Güntersthal is about an hour's walk away, just follow the signs along the paths. It's an ideal little German town, with a small restaurant on the main road where you can stop for some Schwarzwaldküchen and coffee. You can even take the S-bahn straight back to Freiburg.
Conclude your day in Freiburg at Hausbrauerei Feierling for their delicious beer and lovely Biergarten. Their Spätzle was the best I've ever had, in case you're hungry again.
Side note: you could also do all of this with a bicycle and skip the S-bahn all together.