Watermelon - especially good, sweet, flavorful local watermelon - is pretty great straight up. But it also lends itself well to sweet-savory combinations, especially with sour or salty counterpoint ingredients. You can easily freestyle with the general concept, but here are some ideas to get you going. Quantities are basically according to your taste and the amount of watermelon you start with; add seasonings in small amounts until you are happy with the flavor.
In all cases, start with cubed watermelon, seeded if you like.
1. The classic: chopped fresh mint, lime juice, touch of salt
2. Diced tomato, chopped fresh cilantro, lime juice or cider vinegar, salt and pepper
3. Crumbled feta, freshly ground black pepper. Add diced tomato or cucumber if desired.