Football/Soccer
Tuesdays and thurdays at 5 p.m., sundays at 3 p.m. people are playing just for fun at the river Lahn.
Fitness center
We still need to figure out if there is actually a fitness center where you can make a contract only for the time of your visit
Tuesday-Night-Skating
If you have inline skates with you, you can take a special tour through kind of every street in Gießen with many other people every tuesday with music.
http://www.tns-giessen.de/
Slackline
There is a special facebook group where people post when and where they put a slackline between two trees in Gießen.