Trim the ends off the sugar snap peas and cut them in half crosswise, halve the tomatoes, cut the watermelon into 1-inch cubes, thinly slice the onion and place them all in a large bowl. Toss to combine.
Coarsely chop the mint and add it to the bowl them crumble in the Feta cheese.
In a small bowl whisk together the olive oil, the juice of the lemon, salt and pepper. Drizzle the dressing over the salad and toss gently to coat.