There are two things to do with watermelon. The first is simply to chop it up and stick it on a plate. The second is much more fun.

Ingredients:

1 Watermelon, big but not huge.

1 Bottle of tequila (I find Cuervo Gold is fine for this enterprise).

1 Helper (adult). 

 Method:

Find a dish or something, even a cardboard box, that will hold your melon laid down lengthwise. Take an apple corer and make a small hole in one side of the melon. Hold the melon with the hole facing the floor, and have your helper first uncap the bottle, then screw it into the hole. Now carefully rotate the whole thing so the bottle is sticking straight up, and place in your bowl, box, or whatever support you're using. Grin like an idiot as you watch the watermelon drink the whole bottle of tequila. Once the bottle is empty, unscrew and recycle it. Let the whole thing stand ten or more minutes, then cut a large oval out of the upper face of the melon so it's like a big boat. Leave dessert plates, spoons and an ice cream scoop by the melon and let the adults have at it. Guaranteed to make your party memorable. Just make sure (a) the kids don't get in it and (b) anyone who has it has a designated driver or other ride home.