Chooses the pumpkin: Peach! She loves doing the Halloween shopping.
Carves the pumpkin: Mario, while surrounded by Toads and helping them out with their pumpkins.
Gets scared and clings the other in a haunted house: Peach.
Matching costume idea: Peach. They went as Philip & Aurora.
Steals the other’s candy: Peach, 'cause she's playful like that.
Accidentally gets lost in a corn maze: Peach, much to Bowser's convenience.
Tells spooky stories to scare the other: Mario, preferably with a crowd. He loves to tell a good story!
Collects cool-looking leaves: Mario. He collects things all the time. :P