The Obersee, an artificial reservoir and the railway viaduct, very interesting.

Also, if the lake is not frozen, very nice for hiking.

<< Recreational landscape Obersee / Johannisbachtal >>
"The 310 hectare area around the Obersee and the lower Johannisbachtal is one of the most important and popular local recreation areas in Bielefeld. The Obersee, the city's largest water area with 15 hectares, is in season all year round. Water, walking paths, gastronomy, mini golf, playgrounds and many events guarantee a high recreational value.
To the east of the railway viaduct is the Johannisbachtal as a spacious, open cultural landscape with a meandering stream and large pastures in the area of the valley floor and agricultural use on the slopes. From the footpaths around the Johannisbachaue and the Jerrendorf Landscape Park you can enjoy beautiful views over the agricultural valley, the Teutoburg Forest and the two old farmsteads Meyer zu Jerrendorf and Wehmeyer.
The juxtaposition of a wide range of leisure activities and nature experiences as well as attractive walking, cycling and riding trails make the area a varied recreational area, especially for families. "
bielefeld.de/node/4002de.wikipedia.org/wiki/Obersee_(Bielefeld)