Use the chef's knife and cutting board to thinly slice the cucumber into half-moon shapes.
Chop the watermelon into uniform 1-inch cubes using the same knife and stable cutting board, ensuring even, bite-sized pieces.
Transfer the sliced cucumber and watermelon cubes into a large mixing bowl, gently tossing to combine.
Crumbly feta cheese is sprinkled over the mixture, then lightly drizzled with olive oil if desired. Season with salt and pepper to taste.
Toss everything gently until evenly distributed. Transfer the mixture to a serving platter and serve immediately to maintain freshness.