A friend of mine made this for me one summer a number of years ago, and I remember thinking that it was the perfect treat for a warm day.
Ingredients are watermelon, feta cheese, sliced red onion, mint, and dashes of lemon juice and olive oil. Mixed gently. We ate it tonight with a lovely dinner that included steak and stuffed potatoes.
Update: This morning, one of my favourite websites in the whole world, Cake Wrecks, featured watermelon cakes for National Watermelon Day. Read and enjoy!