I love this whole article on Lifehacker. So much, that I am posting it here in its entirety.
The perfect drink dispenser for your summer cocktail parties may actually be a watermelon. Serve punch, juice, or a real fruit cocktail in that giant melon.
For your watermelon-to-keg transformation, simply cut a lid at the top and scoop out the watermelon, then attach a spigot (available at beer making supply companies) via a hole you either drill into the melon or make with an apple corer.
KegWorks offers a watermelon sour recipe along with the keg-making instructions from The Food Network, while the National Watermelon Promotion Board has somewhat more detailed instructions.