Watermelon and mint are two of my favorite summertime flavors — nothing is more refreshing. Plus, this is a relatively hands-off recipe; just blend, freeze, and eat!
In a blender, combine the watermelon, mint, and simple syrup and purée for a few minutes. Pour into a large glass 9-by-11 casserole dish. Freeze for at least 1 hour — you don't want this frozen solid but also not like slush. Use a metal spatula to scrape up the large flakes of granita.