Stir the citric acid or fresh lemon juice into your quart of cold water. Drop the apple quarters in and let them sit for about five minutes. This step is what lets you prep hours ahead without the fruit turning brown and unappetizing on the platter.
Pat the apple quarters dry with a paper towel. Using your paring knife, carve a shallow "mouth" shape into the curved side of each piece. Keep the cut fairly deep so there's room to hold the peanut butter and toppings without them spilling out.
While the apples dry, slice your cheddar cheese or mango into small triangle shapes. These will sit inside the mouth as the "tongue," so aim for pieces about the size of your thumbnail so they don't overwhelm the bite.
Spread a thin layer of peanut butter inside each carved mouth using your butter knife. Press the pine nuts into the peanut butter along the top and bottom edges of the mouth to form rows of teeth. Work quickly since peanut butter firms up a bit once it's been in the fridge.
Tuck one cheese or mango triangle into the center of each mouth so it looks like a tongue poking out. Dab a tiny bit of peanut butter above the mouth and press one or two candy eyeballs into place. This last step is what turns a plain apple slice into a genuinely fun halloween party food table idea.
If you're serving right away, arrange the monsters on your platter and set them out. If you're prepping ahead, cover the tray loosely and refrigerate until 30 minutes before your guests arrive, then bring them out at room temperature for the best texture.