You will love Lake Barkley. It is a large state resort park, indoor and outdoor pools, beautiful surroundings, a huge dining room with panoramic views of the lake and very good food. You've already got your bases covered when you rented the boat and the swimming pools, playgrounds and room to run are the extras!
If you have the opportunity, visit the small town of Cadiz for some shopping/antiquing and the drive thru elk and bison prairie is a short drive away and well worth the $5 per car (not sure if still $5, been a little while)....by the way, the scenic driving, for hundreds of miles surrounding the LBL area and the Trace down the center will lead you to many parks, wildlife areas, learning centers, campgrounds, picnic areas and outdoor paradise galore!! Do some online research of Land Between the Lakes areas, there's so much info I couldn't possibly give you enough here-- you won't regret it! (unless you forget to bring adult beverages with you-- alot of the LBL area is "dry" and you might want to chill out after the kids go to bed!)
Have fun and happy 4th!