When we were growing up, our family went to Sinnissippi Park often. It has beautiful rolling hills leading down to the Rock River and there are some caves at the bottom also. We were not allowed to go to the caves as small children.
The west end of the park was a lookout area on a bluff that allowed you to view the bend in the river and it was always a beautiful area. On the bluff was a series of mounds that later were excavated and found to be graves from the Hopewellen Indian Tribes. The National Historic Park District put them on the Historical Register. Our family still spends time at this park. It is the park mentioned in our book, "Kaleidoscope Memories: Childhood Stories That Celebrate Family Life."
As a little tidbit - author Terry Brooks is from Sterling and has set his book, "Running With The Demon" in this town and the park. You might like to check it out also.