This is probably the best thing to do in Cleveland for free if you have kids. If the weather's good, go down to Coventry Village and enjoy this amazing community playground.
The Coventry P.E.A.C.E (People Enhancing a Child's Environment) Park is also one of my favorite places to go swing on some swings or run around in the grass. What can I say? I'm a big kid myself.
You can find the Coventry Peace Park next to the Coventry branch of the Cleveland Heights Library on the corner of Euclid Hts. and Coventry.