I go fairly light when it is just me/adults, but I still need to bring a bag. I carry my phone, kindle, reusable water bottle, battery pack, sunglasses, and visor, all of which can be put in or clipped on to a small crossbody bag. I also have my id, credit card, cash, and motrin in a small zippered pouch. This is the same bag I pack whenever we go on excursions to zoos, museums, parks, historic sites, whatever.
If the kids are with us, DH carries a small back pack with the addition of snacks/protein bars, more sunscreen, and bandaids.