We made some watermelon oobleck. It's the same as cornstarch and water but with watermelon. We scooped out all the watermelon flesh and put it through the blender and then mixed in corn starch. (If anyone hasn't played with corn starch and water before it's really cool. If you push on it or squeeze it at all it seems solid, but if you pick up a handful it will ooze through your fingers.)