Scarecrow with a Pumpkin: She found scarecrow-inspired clothing at Savers, and cut a hole in the shirt for the belly. She then found orange, black and green facepaint (non-toxic and free of parabens) to paint the belly like a pumpkin. You can add black paint to make a Jack O' Lantern face, too!
Pot O' Gold: Paint a white shirt like a rainbow, and cut out the belly. Use the same aforementioned face paint to paint the belly all black. Cut out gold "coins" from a sheet of foam paper and attach with hot glue to the shirt, and apply some to the belly with eyelash glue. Don a rainbow wig and white leggings to finish off the look.
You can find a few more ideas over at muchachamary.blogspot.com.