You've just reminded me of one of my mama's specialities, perfect for the summer.

Get a large water melon.
Stick in the end of a small funnel at the top of the melon
Every 6-12 hours top up the funnel with some vodka (be as patient as possible, the longer this stage takes the better the melon will taste)

Eventually the watermelon becomes saturated with vodka and you just slice and serve. (You can macerate some basil in the vodka and perhaps dissolve a little bit of sugar syrup too for a more 'cocktail' like experience.)

When I was younger I sometimes used to smuggle alcohol into music festivals by performing a similar operation using hypodermic needles and citrus fruit.