If you have your party before Halloween this is a great way to start it off. Haunt My Yard is an icebreaker activity your guests will love that also provides a great opportunity for a photograph keepsake favor. As a bonus it saves you some work.
Take all of the Halloween decorations you usually set up in your yard and pile them on your lawn. Once the guests have arrived give each guest a single item an tell them they can put it anywhere they choose. After a guest has placed an item they can come back and get another. Continue with this process until all of the decorations are in place. The guests can work together in an attempt to make a cohesive design or just place decorations where ever they think they look good.
Once they have completed decorating take a photograph of the entire group posing in the yard they decked out. After the party print up copies of the photo and send them to the guests with a thank you card for coming to your event.