I buy organic grapes in bulk when they're on sale, then I wash them, put on single layer on a cookie sheet, and put them in the freezer. Once frozen, I put them all in a freezer bag. (The first steps are important so they don't all freeze together in one block.)
Then whenever I want a sweet frozen treat, which is often in the summer, I munch on some frozen grapes. So delicious, cooling, and satisfying!