Castle Rock is a nice park for some day hiking. On the side of the park right by the Rock River, there are some stairs to climb for wonderful views of the Rock River. Down along the river is also a nice shady area with picnic tables. We didn't eat here, but I can imagine it would be a great spot.
On the other side of the road and down a bit, we hiked on some of the trails. They were 1-2 miles each and took us through wood and open prairie areas. One con is that the trail maps seemed somewhat confusing. We ended up on a different trail than we started on and therefore didn't loop back to the parking area we started at. We came across another family on the same trail who were also confused and commented about the signs not being the best, so it wasn't just us. Regardless, it was a nice place to hike and we enjoyed our afternoon,