If you have the time, take a bus to Braunfels (about 30 kilometers north of Frankfurt, next to Giessen).
I was born there, but the point of your visit would be to go to the Foresthaus Guesthaus. My grandparents ran the place in the 50s. It's set in about 200 acers of woods with trails and what not. Braunfels also brews some of the best local beer in Germany.
There's also a midevil castle in braunfels that you can tour.