Pack sunscreen. Pack a few waterproof SPF values, starting at 15 and going up as high as you can. Grab some aloe, too, in case the sunscreen doesn't work.
Book some reading time. Bring reading material, along with sunglasses with reading panes in them. Grab some books you've been waiting to read, and bring more than you think you need -- some of them might be duds. Or, bring a few classics that you read a thousand years ago. You'd be surprised how much fun "Huckleberry Finn" can be at the beach. Throw a few visors in your bag to cut down on the glare.
Pack a few games, such as a plastic flying disc or beach ball. Just make sure to set up the games on the beach away from other sun worshippers. Consider investing in a cheap metal detector -- these can be great fun for a beach vacation.
Take a snorkel for every beach vacation, even if you expect the sea will be too murky to use it -- you never know. Bring at a beach blanket and a few large, but lightweight, beach towels for drying off. If you have room, include a beach umbrella to provide shade for the hottest parts of the day.
Grab a colorful beach tote bag and fill it with some snacks (individually wrapped so sand doesn't invade) and, of course, fresh bottled water. Add a plastic bag with a clean, wet washcloth in case you get sunscreen in your eyes, or just to freshen up after a swim in the sea. Throw in a few disposable cameras instead of bringing along your new digital. Sand can ruin a camera very quickly. Bring hair conditioner, and add it to your hair before and after every swim. This leaves your hair silky at the end of the day after that wonderful shower.
