Sunscreen, aloe vera, lotion, one bathing suit for every 3 days (minimum of 2 suits, one to rinse/dry alternately), same with bathing suit covers, sunhats for toddlers or younger and older if they like, one towel each and I'd take very thin ones that dry quick, water shoes if you will be in a rocky area, flipflops if not, blow-up water toys for the kids, sand shovels (don't really need pails, you can use cups, etc. , collapsible cooler, ipod or walkmans, good reading material for kids and adults alike, disposable underwater cameras from the dollar store for everyone over the age of 4, insect repellant wipes, shorts, tshirts, and sneakers. One or two nice sundresses/outfits for eating out.
Really, it depends whether you will have a car once you get there. A nice beach chair (
Walmart $8) is a must for me. A bigger cooler than a collapsible is nice if there are 3 or 4 of you, more beach toys if you can do Walmart run, etc.