Peel each banana and slice it in half crosswise, so you end up with two shorter pieces that stand upright on their own. Stand each half up on your cutting board with the rounded top facing up, since that rounded end becomes the ghost's head.
Take your mini chocolate chips and gently press two of them into the top of each banana half to form eyes, with the pointed tip facing outward. Add one more chip below the eyes, pointed side pressed in, so the flat side faces out to look like an open mouth.
Peel each mandarin orange completely, removing as much of the white pith as you comfortably can so the fruit looks clean and bright. Set the peeled oranges aside on your cutting board while you prepare the celery.
Slice your celery stalk into four equal pieces, then cut each of those pieces in half lengthwise so you have eight thin strips total. These thin strips become the little pumpkin stems.
Gently press one celery strip into the top center of each peeled mandarin orange, leaving a bit of it poking out like a real pumpkin stem. Repeat with all eight oranges until every one has its own little stem standing up.
Arrange your banana ghosts and mandarin orange pumpkins together on a serving platter, mixing them so the display looks lively and colorful. Serve immediately, since the bananas start to brown once they're cut and exposed to air.