1 frozen banana, broken into 4 chunks
1 heaping cup mini seedless watermelon, diced
2 or 3 frozen strawberries
1 tablespoon light agave nectar (optional but recommended)
Water if needed
Basically throw all your ingredients in a blender and blend until smooth. I recommend making sure the watermelon is closest to the blender blades to ensure easier blending.
Source of recipe: http://veganluvies.blogspot.com/2010/05/watermelon-smoothie.html