This is a park with a damned up lake by Euskirchen. There is a small discovery trail (Ökologischer Lehrpfad an der Steinbachtalsperre) of 3.3 km (about 1 hour) with playgrounds and a sensory trail (Naturerlebinspfad). There are many other hiking trails in the area as well.
There is a restaurant and a grill hut which can be reserved for picnics.
There is also an open-air swimming area in the woods (only open in the summer). Address of the swimming area: Wilhelmstrasse 32-34, 53879 Euskirchen
It's about 45 minutes driving from Bonn.