Make glitter pumpkins! You just give the child a cheap sponge brush and a paper plate w/ some elmers glue. They can paint the pumpkin anyway they want and then sprinkle glitter on to the pumpkin over wax paper. Beautiful! Photo and instructions at Farmer'sDaughter...
Make trick or treat bags. There are a million creative ways. This is a kit from Papersource which is a fantasticly fun party site. The kit was available last year and I hope it will be available again, but it has all the stuff so that the littles can just glue them together. But, you could pre-prep some of this stuff depending on how small your party goers are.
I realize this party is a little heavy on the crafting, but Halloween is a fun time to make silly things. I love these little shrink film pendants. If your party guests are quite small, then outline a pumpkin, witch or ghost etc.. and just have them color. Then, let them watch the magic as the shrink film does it's thing with the oven light on.. It's so fun! This is also a great way to save early artwork. Have a child just doodle, then put their name and date and save in a little box or jar.
image from hobbyloco.com by Cheryl A. Smith @