Using a sharp chef's knife and a cutting board, chop the watermelon into uniform, bite-sized cubes. Place the cubes into a large mixing bowl.
Peel the cucumber and slice it thinly into rounds using the same cutting board and knife. Add the cucumber slices to the bowl with the watermelon.
Crumb the feta cheese with your fingers or a fork and sprinkle evenly over the watermelon and cucumber mixture.
Roughly chop the fresh mint leaves and add them to the bowl, distributing evenly. Drizzle olive oil over the mixture if desired.
Squeeze the fresh lemon juice over the ingredients and season with salt and pepper to taste. Gently toss everything together using two spoons or tongs until well combined.
Transfer the salad to a serving platter or individual bowls, and serve immediately for maximum freshness and crunch.