A few other places that may have some differing varieties of parks amenities (maybe not a great series of walking trails) are Yellowbanks State Park in SE Polk County and the Neal Smith Wildlife Refuge.
Yellowbanks is a picturesque park on the Des Moines River with trails, oak savannahs, indian burial grounds, etc.
Polk County Conservation Board
Neal Smith Wildlife Refuge over in Jasper County has a prairie landscape with a nature center and even a large herd of buffalo and other native animals to check out.
Friends of the Prairie Learning Center - Neal Smith National Wildlife Refuge