If you actually have five full days, Baden Baden is close by. It gets mixed reviews here. We had a nice day there, weather-wise, and enjoyed walking around. There is an easy trail through a park to the next village over, Lichtental. Lichtental had a market day when we we visited (Saturday?). It has a nice feel to it, attractive residences on the side streets, an abbey with small grounds. I'm pretty sure we had lunch there.
I'm giving thought to staying in Gengenbach or Staufen next year. I'll have a car but will also use the train service. Here's a thought, outside of your travel time range, but this should be a relaxing day, and I stand to be corrected about the train service. Take an early train to Donaueschingen (part of the Black Forest Railway mentioned by Lee?), then
A) catch a train to Freiburg (Höllental Valley Railroad?) to spend the day. The core area is flat with nice buildings and, as Tim says, does not have a big city feel to it. There was also a busy outdoor market around the cathedral on the day we visited.
B) take the train to Bad Krozingen, then walk along a creek to the fabulous little village of Staufen – it's a flat walk of about an hour, surrounded by farmland once you leave Bad Krozingen. This was one of my wife's favourite vacations days of all, so we had to repeat it.
The falls walk is the best thing about Triberg for me, not overly keen on Triberg itself. Re Schiltach, the day I was there the Café Bachbeck Konditorei had a Bienenstich cake which is nothing like I've tasted anywhere before. It was loaded with cream, hardly any sponge, had a crispy top.