If you're willing to explore, Southwest Texoma is full of great places to cool off.

Take the Quartz Mountains North of Altus. You wouldn't normally look to the North and think "I bet there's a lake atop the peaks of those mountains." but believe me, it's there.

It's also a great family place. There's a swimming beach, fishing docks, and plenty of trails to be explored. Go-carts, mini-golf, and sometimes a few amusement park rides to tide little over after the sun goes down.

You can bring a tent and camp, rent out your own cabin, or stay in luxury at the Quartz Mountains Resort. I know this reads like a sales pitch, but it's not. People just tend to not know about it, and I just was lucky enough to spend a few great Summers there growing up.

Pro-Tip: Blair, OK (town near the lake) has a great steakhouse. Tip well.