The Cuyahoga Valley National Park is near by and free to enter and explore. There are fees for some activities, but just to visit is free. http://www.nps.gov/cuva/index.htm
If you have kids between the ages of 5 and 12, as a ranger about the Junior Ranger Program. It gives children an opportunity to learn about the park in a fun way and then earn a Junior Ranger badge and a certificate upon completing the activities.