The first thing about having a party at the beach is to make sure that it is not only clean, but that there are also restrooms, a grassy (or concrete) area and a shaded area near your party spot. If there is not a shadey spot, then you should use a canopy to make one.
Decide on what type of foods you will be having. Are there grills you can cook on? Do you need any of the food refridgerated? Are you going to travel back to the house to eat? You get the point.
If you have decided to bring food with you, then have plenty of cooler on hand as well as garbage bags. Make sure you have a box of wipes and some paper towels.
To make a quick table if the beach has none, use a piece of plywood and cover it with a table covering.
For the invitations, inflate beachballs and put all the info on them with permenant marker and deflate. The invitee will have to inflate for their invite info. Make invitations to look like beach balls or the beach or a palm tree.
Bring a radio for music and have a limbo contest. Have a sand castle contest. Have a balloon ball toss.
~~~~~~~~~
I still have ideas but do you want them for the actual "beach" or a "Backyard Beach Party"
Let me know and I will get on it