Use apple corer or paring knife to cut three small holes about ½ to ¾" wide to fit celery legs on one half of the watermelon.
Put skewers close to top through the one half of watermelon with holes. You may criss cross skewers if desired. You may wrap melon halves in cling wrap and place in fridge overnight.
Skewer watermelon balls with other fruit you have available.
When ready to serve, attach celery legs, create lid handle by cutting out 'C' shape out of zucchini or cucumber. Attach 'handle' with skewers or toothpicks.
Use dark blue colored fruit for coals and add to bottom of grill.
Use skewers to attach grill lid.
Place fruit skewers on and around grill.
Notes
Based on recipe from SheKnows - https://www.sheknows.com/food-and-recipes/articles/1004269/watermelon-grill-with-fruit-kabobs
Recipe by say yum!™ at https://www.sayyum.com/watermelon-grill-centerpiece-heats-up-your-summer-bbq/