What is a good list of things for a Halloween scavenger hunt party?

Depends on the kind of scavenger hunt and the age group involved.
If it's younger children and they're items hidden or held around a smaller area then there are lots of cool things. Those tiny pumpkins from the grocery store you can hold in one hand, rubber mice, plastic spiders, fake eyeballs, fake pirate treasure, etc. Maybe throw in old costume props if there are people or parents willing to donate items that they don't use anymore. Kids love finding cool toys. A pirate's eye patch or plastic sword, those rubber witch fingers you can put on the ends of your fingers, a goblin's ears. Things they can play with or wear when they find and you could mostly buy for a few bucks at your local Halloween store.
If we're talking older kids or people who may be driving or walking around the neighborhood asking from house to house for items they might have to be things people are more likely to part with or have around the house so it's a little trickier. Maybe have a list of simple items that they have to combine to create something when done and that's where the Halloween theme comes in. Simple things like rubber bands, paper clips, plastic cups, an orange crayon and other things people are likely to have on hand and give away. Before turning in their items maybe they have to make something Halloween related out of the items given. It would be more challenging for an older group and require creative thinking and group participation as well as keep a Halloween theme. Given cottan balls, kleenix, a marker, and a rubber band or piece of string someone could make a ghost for example.