Cut open the mini watermelon in half. Using a melon scoop or ice cream scoop, dig out both sides of the watermelon. (Keep the scraps!)
Taking the pineapple chunks & watermelon chunks (scraps), alternate on a skewer. Repeat 3 to 4 times.
Using an apple corer, make 3 holes on the bottom of one side of the watermelon. Take the 3 celery stalks & place in these holes in order to make the stands of the grill.
Fill the bottom of the watermelon with the blackberries, to represent charcoal.
Carefully push 4 to 5 skewers (depending on size of watermelon) through each side to form the grill grates.
Lay the pineapple & watermelon skewers on top of the "grill grates".
If desired, use the other half of the watermelon as the grill lid.