A Halloween-themed activity we did last year, which the girls could do much better this year as 3-year-olds, was decorating craft pumpkins with playdough. This year we used black playdough and accessories such as googly eyes, yarn, feathers, and hats, and they turned out great!
We opened up a fresh container of playdough, so it stuck to the pumpkins really well (if you make your own, make it on the moister side so it doesn't fall off and frustrate the kids). The girls (38 months) didn't require much instruction to figure out how to make the faces.
We used yarn and feathers for the hair, but I can think of other accessories we can use when we repeat this activity.
L and E played with their pumpkin faces for a long time. They changed their expressions (sad pumpkin, angry pumpkin, etc..), and made up stories about them. We'll probably be repeating this easy activity at least every week until Halloween since the kids enjoyed it so much.