The River Lahn is one of Giessen's best assets. A well-marked cycle path runs along both banks and connects the city to Marburg to the north and Wetzlar to the south-west — both reachable within a half-day's cycling. In summer the riverbanks fill with students; there are several spots where locals swim, particularly downstream from the city centre.
Canoes and kayaks can be rented from operators in and around Giessen, and paddling the Lahn through the valley is a popular day trip. The river is gentle enough for beginners but varied enough to stay interesting. If you prefer two wheels, the Lahn cycle path (Lahntal-Radweg) is well-signposted and can be done as a multi-day tour all the way to the Rhine, with accommodation and bike repair stations along the route.