hmm, this is a good question. I also sent out these type of invites last year(mine weren't as good looking as yours).
How about the message in a bottle invite. I think they sell those on oriental trading.
Or get the little box or envelope and make little witch brooms. I've been playing around with making little witch brooms on my blog(http://getsboredeasily.blogspot.com)
maybe with a tag line that says "Fly over to our house on October 23rd for a frightfully good halloween party" or make up your own tag line I'm not very good at them.
A CD with the invite printed on the label with pictures of last years halloween party. The tag could say something like "Come see what all the excitement is about...You're invited to the Johnson's Annual Halloween Party on October 23rd.
A picture of a witch with a picture of your guests face on it...Tag...It wouldn't be a party without you! Please plan on flying over to our house on October 23rd..
A bag or box full of wiggly eyes that says "Eye think you should come to the annual Halloween party on October 23rd"
A pocket mirror with a tag that says "Open this to see who is invited to our halloween party this year."
OK, I'm getting a little crazy...but good to get some brainstorming going..