The watermelon salad is one of the most refreshing summertime treats. The cool, crisp, sweetness of the watermelon pairs beautifully with fresh mint and tangy feta cheese. But what then?Well, on one hand, maybe that's it. Some baby greens and a drizzle of balsamic vinaigrette might be all you need. You could even add nicoise or kalamata olives. In fact, you may well discover that your watermelon salad has become another kind of salad altogether. Like a greek salad with watermelon. One thing we know for sure is that some thinly sliced red onion is going to add a welcome piquancy to the salad. But you know what? You could substitute thinly sliced fennel for the onion and get a different but also completely wonderful result. One suggestion we will make, however, is that you not add any other fruit. We know, we know, tomatoes are a "Fruit, " And so are olives. But what we mean is, don't go adding grapes or berries or some other kind of melon. You're straying into fruit salad territory there, which is a completely different animal. If you can and there's room in your fridge, it helps to start with a chilled watermelon.