First of all Decorate
You have to set the scene, after all. Harness your younger ones' energy by sending them into the yard to find some rocks they can wash and scatter around for that beachy feel. If you're into painting rocks, decorating some with fish can also be fun. If you don't have craft supplies they're available on line. A plain candle can be dressed up by some glued on shells, (Which can also be ordered on line. I just bought some and they're great!) Younger children can draw beach-themed pictures which you can hang in the window or string on your back patio.
You have to dress for the beach, right? Have everyone in the family pick the beach clothes for each other from someone else's closet. Mom's bikini top for Dad, perhaps? Dad's old Hawaiian shirt for little Junior? You'll want to be sure to take some selfies to post! Also, if you have crafters in the family, you can buy charms on line for a reasonable price and make some jewelry to wear to the party.
You can't have a beach party without food. If you've got teens who like to cook put them to work making some good, old-fashioned potato salad and deviled eggs. If you don't have a grill for hot dogs it's okay to cook them on the stove top.
For dessert, how about an ice cream octopus? Fun and easy for the younger ones to make. I've got the instructions on the recipe page here at Sheila's Place. (And you know you want to check out that page, too, right?)
Any number of activities will keep different age groups happy. Younger ones will enjoy playing Hide the Shell. You can do this with a real shell or a small picture of a shell. It's like the old Hide the Button game. Send everyone from the room (or the back yard, if you're out there) and hide the object. Bring the gang back out and let them search, letting them know if they're getting warm, hot or cold as they get closer or farther away from the object. Whoever finds it is the next one to hide it.
Surfing doesn't have to involve water. If you've got an old plank sitting around from some previous project you're in business. The gang can air surf. Have two muscled members of the family hold each end of the plank a couple feet above ground level, then put your surfer on it and jiggle the plank. Have a contest to see who stays on the longest.
How about a game of Beach Bingo? Make your own Bingo Boards. (Several websites offer free templates.) Replace the word BINGO with BEACH. You can use Skittles, M & M's or chocolate chips for your markers. Whoever wins gets out of a household chore for the week, gets a double allowance, or ... whatever prize you think your family will enjoy.
You can play backyard beach volleyball even if you don't have a net. Simply make a line with rope and let the ball fly where it will.
End your party with a story. Let someone start the story with a beach-themed beginning, such as Once Upon a Time There was a Giant Squid. That person can stop the story wherever he or she wants, leaving the next member of the family to continue the story. That person, too, can stop anywhere in the story. Last in line has to finish the tale.
JUST LIKE OUR GRANDPARENTS AND GREAT GRANDPARENTS DID, we can manage to make the most of life in difficult times. Hopefully, if your family can't go to the beach this will bring the beach to you!