Place the watermelon, simple syrup (if using), and lime juice into a blender and pulse until blended. You want to make sure there are no chunks in the mixture, but do not overblend, or else you will warm the drink too much.
Pour in a glass and serve immediately, garnished with fresh mint and a lime slice. Enjoy!
*Simple syrup is just equal parts sugar and water, boiled together until the sugar has dissolved. Here is my recipe for Strawberry Simple Syrup. You can add it here as is for an additional fruit flavor, or leave the strawberries out. You may also leave the simple syrup out altogether, if you want only the natural sweetness of the watermelon