A little hint for when you're picking out your watermelon, look for the sitting spot! If it is white, then it is not ripe enough yet and was picked off the vine early. The more yellow it is the more flavorful it will be! I tried out this technique today when picking my watermelon and it was perfectly sweet.
1/2 of a mini seedless watermelon, rind removed & cubed (about 2 cups cubed watermelon)
1 tablespoon lime juice (I just used the juice of one lime, I love limes!)
1 teaspoon minced fresh ginger
4-5 mint leaves, ripped into small pieces
10 or so ice cubes (more or less depending on how icy you want it and how big your cubes are)
Place all ingredients in a blender and puree until smooth and slushie-like. Taste and add more mint, ginger, or ice if desired.
Adapted from http://www.inthiskitchen.com/