Swimmers, towels, food, drinks in an esky, an umbrella, sunscreen and sand toys .. that's about it. We don't often spend a whole day at the beach because the kids are too little to be out in the sun for a whole day.
My car is always packed with everything for the beach, I don't tend to take things out of the car since we spend so much time at the beach - beach chairs, boogie board, balls.