Something we have enjoyed lately are watermelon slushies. These are super easy to make and very co-o-o-ld to cool you down on a hot day. This is a recipe we really just make up as we go, so there are no real measurements.
Cut up watermelon into cubes. Most watermelons are seedless nowadays but if you have one with seeds, make sure you get those out. Put your watermelon in your blender. If you are looking for amounts, probably 1/3 full is good. Then add some ice. We start with a bit less than the amount of watermelon we put in. Blend that together. Continue to add more ice until you have reached a thick slushie consistency (no longer runny). Spoon into glasses and enjoy! Oh, and a word of caution - it's a good idea to scoop instead of pour the slushi into the glasses... take it from me, you will end up with a mess on your counter if you don't. :-)
Though we haven't tried it this way yet, you could experiment with flavors too - add a little lemon or orange juice to give it some zing, or maybe mint?