Tedesco Environmental Learning Corridor in Ames is a great place to walk around and for kids to explore. The main paved trail is 1 mile long and sort of loops through the area. To see more though, you'll want to explore the secondary permeable trail which adds another 1/2 mile.
The secondary trail has a creek crossing, so if you attempt it with a bike, you'll be carrying the bike across boulders (as shown in one of the photos below).
The map shows restrooms available at the Iowa State University Economic Development Core Facility adjacent to the parking lot, however I'm not sure what hours the restrooms are available, or if the building is open on weekends.
The paved trail connects to the Vet Med Trail, if you're looking for a longer ride. Provisions Lot F has great food and is along the route.