Chop the watermelon and arrange it on a platter. Drizzle with olive oil. Sprinkle with feta crumbles, basil or mint, and kosher salt. Add lemon zest or black pepper if desired.
Serve immediately. If making ahead, refrigerate the cubed watermelon separately: it releases a large amount of water after it is cut. Then assemble the salad right before serving. Leftovers store for up to 2 days.