There is a pretty lake in Titisee. You can rent row boats or paddle boats for about 6 euros an hour. I particularly enjoyed watching my boyfriend display his strenght at rowing while I sat back and enjoyed (all) the views. I tried a little but the ores were a bit heavy for me so I didn't last long. You can also take a power boat for 8 euros an hour, but they don't seem to go very fast....
The area also has lots of restaurants and shops. Seems like there's a trail around the lake, but we didn't do it because we planned to hike in Feldberg. (See my tip on that.) Feldberg is close to Titsee so you could also do the two in the same day like we did.
We walked up to the top of Feldberg, the highest mountain in Schwarzwald, and then walked down to the lake, Feldsee, and back up to the car park (by Nature House). From top of Feldberg, you can look down and see Feldsee. The hike down to the lake from the top was one of the nicest hikes I've done also. It takes you through the forest and the views near the top are nice. The trails aren't easy because it's not paved or flattenned, so there is not much traffic, if at all. Once down at the lake, you can walk around it. Be sure to get a map (you can get one for 50 euro cents) of the area before you go up to the top. The whole thing took about 3 hours.
There are lots of cuckoo clocks in Schwarzwald. There are also many Cuckoo Clock House. The largest one is now in Triberg. The second largest (used to be the largest) one is also just outside of Triberg. I went to this one. It costs 1.20 euro to get in. Once in, you get an explanation of how the cogs work, and you can go out in the back and take pictures of the cute house. You could also walk out to the main road with your zoom camera for a photo. The cuckoo cuckooing on the hour isn't very exciting at this one though so don't hold your breath for it.