One of the best socials that I was part of was a haunted tour
of our campus. We did this with one of our local sororities and we all had a blast.
Our social chairman wore a nice suit and a devil's mask with a plastic pitchfork to fit the theme. We then toured the campus and heard all the ghost stories. This worked really good for us because we have a lot of activity on our campus.
If your campus doesn't have any crazy ghost stories there are many websites that have lots of information you could reference and find some local sites!