Watermelon, I knew, would be safe bet.
But a "watermelon cake" turned out to be absolutely calorific!
A little hard to push candles in . . . but the birthday girl insisted on at least one.
The "cake" sliced nicely and was enjoyed immensely--all without a gram of guilt.
I wish I could say I invented the Watermelon Cake recipe, but there's actually several on the Internet. My only advice is that when frosting the shaved melon with whipped cream is to keep plopping dollops of the cream "frosting" on top and ever-so-gently push it--or let it slide--down the sides.
Contrary to what some recipes say, the cake doesn't need to be served immediately. I rearranged the shelves in the fridge to accommodate the tall cake plate, and it kept nicely until party time.