Here are some of my favorites. As a big fan of tequila, many will involve this fine golden liquid.
blender full of frozen watermelon balls
two limes, juiced
half cup triple-sec
tequila to taste (cup?)
water as necessary
A day or so before, melon ball up a watermelon and put it in a gallon freezer bag in your freezer. On the day of making, fill your blender with these balls and add the rest of the ingredients around it. You will find that the water is necessary as the melon balls themselves will end up being too thick, even with all the alcohol. Blend to taste and consistency. Best drank through a straw from a margarita glass on a hot summer day.