Here's my take...if it gets thrown at a Halloween parade, it's not a good trick-or-treat item.
Therefore, at least any medium sized candy bar is decent, and a full size one is just super.
I suppose it depends on how many kids there are in the 'hood and how much you want to spend, though.
The random stuff that you wouldn't normally buy is best...like Butterfingers, Mallo Cups, Mounds, 5th Avenues...stuff like that. The kid will want to save it for last because it's a special one.
(Oh, and as far as the stuff thrown at a parade goes...that means that pencils, erasers, rulers, political buttons, and corn cobs - yes, we asked someone on a float to throw a corn cob at me and they did - are not good items to give out - I've had them all thrown at me before at parades.