OK, I'm just going to start by saying, this is one of the coolest things I have ever made. I know it's not that difficult, but it's just such a neat idea!
My mind is spinning.
How many other fruits can I add Jell-O to?
Oh the possibilities are endless.
Everyone beware ...
I may get nuts with this whole Jell-O in fruit thing. 

I made this on the Hallmark Channel here: Jessie Jane makes Jell-O watermelon

I also made this for Brooke Burke's ModernMom.com here: Jessie Jane makes Jell-O watermelon

And here with Queen Latifah: Jessie Jane makes Jell-O watermelon

Jell-O Watermelon

Ingredients  (Recipe is for Half of one large watermelon)
1 Watermelon
4 large 6 oz boxes red jell-O
3 cups Hot water; 2 cups cold water (DO NOT follow the water instructions on the Jell-O box)
1/4 c. mini chocolate chips (These are for the seeds - you can leave them out if you wish)

Directions
1. Cut your watermelon in half and scoop out half the melon with a melon-baller. Make sure to get all the way down to the white rind. 

2. In a large bowl combine 4 large 6 oz boxes of red Jell-O with 5 cups of water. DO NOT use the amount of water they specify on the Jell-O box. This recipe is different and your Jell-O needs to be a little more firm.

3. Pour the Jell-O mix into the melon halve and refrigerate for 4 hours or overnight. TIP: Slide a soup bowl under the melon so it sits evenly in the fridge. (If you are using the mini chocolate chips wait about an hour until the Jell-O is a little firm then press the mini chocolate chips into the Jell-O with a thin wood skewer.)

4. When Jell-O is set, carefully cut into slices. Do not use a serrated knife. Enjoy. 

TIP: If you want to add Vodka to this recipe follow the ingredients below:

1 small (or mini) Watermelon
1 Cup of Vodka
1 Cup of Boiling Water
3 oz Cherry Jell-O or Strawberry Jell-O
1 Envelope of Knox® Unflavored Gelatin