Grand Trunk's Parasheet is a great solution. They are super light weight at 20oz (less for the single), come in a handy attached stuff sack, and easily fit right in your beach bag. They come in a rainbow of colors and single and double sizes. I'd opt for double as I think bigger is better in this case, though the single is more compact and would be perfect for kids or those who want to minimize bulk even more.
The blue is gorgeous and soothing, like your own portable oasis. Sand shakes off with ease, so you don't have an accumulation at the bottom of your bag the way you do with towels that have been on the beach. It also dries very quickly. Pockets are sewn in the corners and sides to fill with sand and weigh it down so it will stay flat and won't blow away, and there are stake loops too. It's perfect for picnics and no worries if you spill food on it, you can just throw it in the wash when you get home.
Grand Trunk has a lot of innovative gear, and they're an eco minded company. Find the Parasheet and other travel necessities HERE.