If you want to do a nice trip starting and ending in DUS I would recommend to travel along the rhine all the way down to Switzerland and back up. As the other mentioned you can stop along the way at the following places:

Cologne, Bonn, Rhine Valley Gorge (with tons of castles, small villages, great wine areas, a world famous monastery Kloster Eberbach), Mainz, Worms, Heidelberg, Black Forest, the Rhine Waterfalls, Basel, etc.)

August is the perfect time for Germany.

Cheers
Christoph