The Gathering Place is an amazing park in Tulsa, Oklahoma which has the most epic playground I've ever seen. I'm so glad we allowed two days to play there during our spring break trip. There's no admission fee for the park, but there's a fee to go into the new Discovery Lab at the park.
When you first arrive, I recommend stopping in Williams Lodge to pick up a map. The lodge has restrooms, two cafes and a great area on the second floor to relax and take a break from playground adventures.
My other recommendation is to have a plan if kids get lost. My boys were wearing Gizmo watches which was very helpful in keeping track of them, however we failed to remind them to stay put if they got lost, so we had a panicked child who we could talk to, but couldn't find because he kept moving.