First urge is to go with waht Non-Gognomia said (YUM!), but I went to a nice house party about a month ago and had my socks blown off by the following:
A guest who brought a fruit salad decided to make it more lavish (and more dessert-ish) by scraping out he watermelon she had melon-balled. She reduced the pulp with a modest amount of sugar to a syrup consistance, then added a not-so-modest amount of cointreau- or maybe it was Gran Marnier- something orange and alcohol-laden. The fruit salad, mostly stone fruits was tossed with this divine syrup.
A few days later, while eating Tapas, I was served some serrano-wraped shrimp served with a melon-fig salsa- the sauce was mist likely a pineapple/cantalope reduction, but a watermelon reduction would work well, too.
You could make a watermelon gazpacho, too- very refreshing.