Easy and adorable cake decorating idea! Fill a cookie cutter with sprinkles, it doesn't have to be a number. Just pick it up! (You can make a shape that's "faded" at the edges by filling the cookie cutter up to the top and lifting it up quick.
"Birthday cake made of fruit" No. Not a cake. Just a big slice of watermelon with some pretty fruit on it. And if you try to serve this to me as my birthday cake, clearly you are asking to be ridiculed for the rest of your natural life.