One of the best trick-or-treat traditions in the area is Living History Farms‘ annual Family Halloween. I’ve visited it a few times over the years (some friends of mine used to vend “sugar coated kettle popped corn” there, long ago). You can see some old photos over at my theme park photo site:
http://misc.disneyfans.com/OtherPlaces/Iowa/LivingHistoryFarms/index.html
We visited it this past Sunday right as it opened. Here is a video montage of the trip: