One of my favorite books, How Many Seeds in a Pumpkin?, is perfect for this time of year. Children estimate how many pumpkin seeds are in pumpkins of varying sizes. In order to find the number of seeds, children count by 2s, 5s, and 10s, and learn that the smallest pumpkin actually has the most.
Maybe you'll want to read it and do some of the "Pumpkin Problem Solving" or "Fall Math Activities" over at Mathwire...one of my FAVORITE math idea sites! ;)