All of the above posts are good advice, also don't overlook Museum of the Red River in Idabel. Good collection of Indian Artifacts and a complete Acrocanthasaurus dinosaur skeleton.
During the heat of the day, take a walk around the Forest Heritage Center in the park, if for nothing else than to admire the way the hallways follow the slope of the land.
First class parasailing operation on the lake.
Take a drive up HWY. 259 toward Smithville, or all the way to Heavener. Mountains get bigger and people get fewer as you head north. If you get close, be sure to visit Heavener State Park. Whether the Viking Runestone is real or not is a debate, but regardless, the setting is beautiful.
Just guided two days there, trout fishing is OK, but not great. But, it never is in August and early September.
Based on cabin rentals and the number of travel trailers I met driving home yesterday evening, crowds will be heavy.
Be sure to go back in the late fall, when the trees are turning and the morning air is crisp, there is not a prettier place in the country. Trout fishing really picks up in mid-October.