Two hours will go by quickly. If it were a sleep over, the movie would be fun, but with only two hours, decide on activities instead.
I love the outdoor scavenger hunt in the dark idea! I envision doing that some day! Thanks!
Painting pumpkins sounds great. Getting the "baby" sized ones for 3 for $1 should be fine to keep the cost down. Remember that the stem can be used as a nose, or the bottom can be used for the face. Aim for creative.
Build a scarecrow with straw and old clothing.
Is this a costumed affair? Make sure they all know. Send them trick-or-treating and see what happens? (One of my students said that he once went trick-or-treating in July, and actually got candy from his neighbors!)
If it isn't costumed, they could do spooky face painting, their choice, or have ideas to pick from a hat, like "hissing cat," or "ghoul."
They could play Halloween charades.
In my day we used the Ouija board and thought nothing of it. Today, my first reaction to reading the suggestion was, "Oh, some parents won't like that idea." Expect a phone call if you follow through with that idea.
How close is the local cemetery? That's the place for the scavenger hunt or a game of truth or dare. Or hide and seek. (which, I suppose, could result in a phone call as well.)
Flashlight tag.
Jump in the leaves?
Roast marshmallows over a bonfire or fire pit and tell ghost stories.
Some kids have no idea how to tell stories anymore. Find an engaging way to give them some pointers and model a story before you send them to do it themselves. Perhaps they could do it in pairs.
My daughter is a social one herself, and she would have definite ideas about what she wants to do. I bet yours does as well. Perhaps she'd like to invite that one special friend to help with the set up, as the decorating and baking would be fun as a preliminary activity.