I think the dinner in a pumpkin idea is fitting. I would do the stove prep the day before, if poss, and then cook it/heat it in the oven before the party. Or I would do it all on the stove the day before, and just reheat it and serve it in the pumpkin for the party. Any soup would be nice for Halloween. If you want it to work with a theme you could just rename it Great-Aunt Edna's Veggie Brew or something)
You could also do chicken salad sandwiches, and make the chicken salad part before, and slice the rolls, and then have people make their own. And that would go nicely with fruit/veggie trays and be filling for an adult. If they make their own sandwiches it's an easy leftover meal. Since you can make fresh sandwiches with the salad later. (I use small chicken slices or shredded chicken, red or green grapes sliced, almond slivers, a little chopped celery, salt and pepper, mayonnaise, and craisons are a nice touch if you have some.)
I recently went to a child's b-day w/ pizza for the adults and it was great; there were a few appetizers (mostly candy) and then there was cake and ice cream, and it went well. If she had prepared more adult food it would have been excessive, because the pizza was a good brand and there were so many different choices.
Good luck!