Position the pumpkin with the flat side as the front of the house. Cut the top off the pumpkin at an angle so that the lid will fit securely. Clean out the seeds and membranes with an ice cream scoop.
Near the middle and slightly up from the bottom, carefully cut out a rectangle for the door. Cut a square on each side of the door to make two windows. Remove the door and each window in whole pieces.
Trim the window and door pieces to about 1/2″ thick by carefully shaving the inside. Cut and remove individual windowpanes. Replace the windows in the pumpkin, allowing them to jut out ever so slightly. Position the door on the pumpkin as though it were just being opened.
Place the pumpkin on the plywood and the can in the bottom of the pumpkin. Use broccoli and lettuce leaves for shrubbery. To make the tree, stick several toothpicks into the bottom of the pineapple top, to stabilize it in the top of the paper towel roll. Wedge the tree securely between the pumpkins.
Make a car from the potato, using carrot slices for wheels and gumdrops for headlights. Make ghosts by tying facial tissue over round suckers with ribbon or black string licorice.
Just before your guests arrive, fill the can half full of hot water and add dry ice.