Pumpkins are an excellent starting point for lessons in simple shapes. Start with several full sized pumpkins per small group to represent a round shape like a circle or oval, and provide the toddlers with pieces of sticky-backed felt cut into primitive shapes, such as triangles, squares, rectangles, and crescents. Have the students decorate their pumpkins and gently direct the activity by suggesting they use certain shapes for certain uses, such as using a triangle for a nose. Be sure to point out when a student has used the shapes as you suggested, showing praise.
Pumpkins represent both the bounty of the Thanksgiving harvest and the silly-spooky face of Halloween. Start with several laminated cutouts of pumpkins and jack-o-lanterns (each with double sided tape on the back) and several large laminated posters of holiday themed imagery, some representing Thanksgiving and others representing Halloween. Challenge the students to decorate the posters using the cutouts. Encourage them to place the cutouts creatively while underlining the differences in the two holidays. Wear a costume appropriate to the season, if you have it.
Pumpkin decoration is traditionally considered too dangerous for toddlers because they are too young to handle a knife safely. However, most preschoolers can handle poster paints, water colors, or water-based markers with little supervision. Provide each student with a mini-pumpkin and a collective source of child-safe art media and direct them to decorate the pumpkin for their Halloween celebrations. While it is a good idea to have a few jack-o-lanterns as models for those who need direction, you should not discourage the students who seek different creative paths. If they want their pumpkin to look like a bunny, let them.
The head-shaped nature of pumpkins makes them an ideal choice for face-style decorations such as jack-o-lanterns, but they also work well as chubby exaggerated bodies for dress up art projects. Have students create a "harvest buddy" using a pumpkin and other harvest themed elements, such as straw for hair, or a "pumpkin pal" which relies on more traditional dress up elements such as bits of cloth and felt. In either case, use stick or liquid glue to adhere it to the pumpkin.