For my son's 3rd birthday Luau party, I made Watermellon Jello. Well, technically I used Strawberry jello, (but you can use any flavor as long as it's red). But all I did was cut a watermellon in half, scoop out all the fruit down to the rine in both halves. Fill it with jello, and let it set up half way and then I sprinkled mini chocolate chips on top of each and used a ka-bob stick (you can use a chopstick too) to stick the chips down into the jello. Let it set up the rest of the way and cut and serve like you would a normal watermellon. It was a HUGE hit with the kids! (And the adults).
Make sure you dry out the watermellon rine before pouring in the Jello. I used a papertowl to get it really dry. Also very important: For each half I used 4 6oz Boxes of Jello to only 5 cups water.
I found this idea HERE! At Gourmet Mom On The Go!