I could have called this a vomiting watermelon, but I wasn't sure if that would have been the most appetizing idea. Then again, this fruit monster is throwing up watermelon and pineapple.
Cut the bottom off the watermelon to create a nice flat stable platform. Next you will have to cut-out the mouth to give you enough of an opening to scoop-out all the watermelon.
Cut the watermelon pieces in squares and do the same with the pineapple.
Notes
You can add any other fruit that you like. You probably will have too much fruit to decorate everything. Just keep it available in a container and add when needed.