For the salad: Using a melon baller, carve out balls of watermelon and put in a large bowl.
Remove the basil leaves from the stems (and throw the stems away). Tear the basil leaves into 1-inch pieces.
Cut the cherry tomatoes in half. Cut the feta cheese into 1/2-inch cubes. Put everything in the same bowl.
For the dressing: Add the olive oil, vinegar and salt and pepper to taste. Gently toss your tomato feta melon salad and serve right away.