Many of my favorites have already been mentioned, but I have a couple of others.

Camping clothesline and clothespins. Don't always need the line, but the clothespins come in super handy. They keep the towels on the railings on a windy day, but beyond that I use them to clip things on to my beach chair (towels, hats, cover-ups, etc) and I can close and store the leftover half bag of cookies or chips.

Cheap plastic cutting board. Great to cut up fruit, makes a good plate for the small kinds of thing you might think to eat in the room (cake, sandwiches, etc). My DH likes to eat on the bed (which he wouldn't dare at home) and the board makes a good crumb catcher. I normally also bring a set of cutlery for each person. I also like to have a stack of solo cups for drinks, and if needed, half filled with sand they make a decent ashtray. Smoke 'em if you like, but please keep the butts of the beach.