I take a small crossbody bag into the park with me. It's the same kind of bag I use in my regular life. That way, if I want/need something, it's right there and I do not have to go back to a locker. Also, my water bottle has a carabiner clip on it, so I can clip my water bottle to my bag. It means I can carry my water bottle hands free and have easy access to it without taking a bag off or opening a bag.