Gengenbach is an especially charming and well-preserved town.
Not as famous as Triberg, but we found the Geroldsauer Wasserfall to be quite rewarding. It is an easy 2-mile loop through the valley and along a creek. Trailhead from Malschbach. But definitely Triberg is the "must see" for waterfalls in the region.
Mummelsee can be kitschy fun if you like Black Forest ham. What we thought was fog over the lake (large pond really), was actually smoke from them smoking the hams next door. The lake has a gruesome fairy-tale/folk legend associated with it, which makes for lots of souvenirs. (Again, kitschy, but also not a typical stop for most Americans, I think.)
During your stay in Freiburg, I definitely recommend seeking out the Feierling brewery's biergarten. Terrific fun, beer, food, and music. The town's cathedral and Augustinermuseum should also absolutely be on your itinerary while you are there.
Finally, it might seem like a tourist trap, but the Trinkhalle in Baden Baden (which is worth seeing anyway) had a great cafe/wine bar that honestly had the best Black Forest cake of our trip.
I think I've given you more than three nights' recommendations, so I'd say there's plenty in Germany by the time you visit the ancestral village too. Save France for another day...