Beach towel that doubles as a blanket (sand-resistant, packs small)
Beach bag or tote you can rinse out
Umbrella or pop-up shade
Refillable water bottle
Cooler or insulated bag
Sun and skin
Sunscreen (reapply every 2 hours)
Sunglasses
Hat or visor
Lip balm with SPF
Aloe for after
In the water
Swimsuit (worn or spare)
Water shoes or flip-flops
Goggles
Dry bag for your phone
Floats or pool toys
Snacks and comfort
Easy snacks that survive heat
Extra water or electrolytes
A book or something to read
Small first-aid kit
Baby powder (gets sand off skin fast)
For the ride home
Second towel or the blanket side to sit on
Change of clothes
Plastic bag for wet things
Trash bag (leave no trace)
Do not forget
Phone charger or power bank
Cash for parking
Keys in a safe spot
Hand sanitizer or wipes
Starred items are the ones people forget most. The right towel does double duty as a blanket and a place to sit for the ride home. See the one I tried at Jen Tried It.