We use an old Jansport backpack and bring: Sunscreen, chapstick, ibuprofen, some snacks for the kids (goldfish crackers, granola bars, fruit leather, etc.), hand sanitizer, a few Pullups, package of wipes, and camera.
If we're going at a chilly time of year or when it gets chilly as the sun goes down, we pack the kids' fleece blankets, sweatshirts, mittens/hats, and long johns (if they're not already wearing them under their clothes).