This smoothie is really good:
1 1/4 cup frozen watermelon (cut in small pieces before freezing)
1 large frozen banana (slice before freezing)
1 cup soy milk
Blend! It is pretty thick, so unless you have a Vitamix, you might need to stir it a little to get it going. I do this while the blender is running. Of course, be careful not to hit the blade.