If your costume skills are top notch, there's plenty of parties in the nightclubs around the strip with some serious cash prizes (usually $5-10k with like third place still $1k). The Fetish and Fantasy Ball (
http://www.fetishandfantasyhalloweenball.com/) been at the HRH for a long time now is something.
People watching is always fun on the strip. The costumes for Halloween weekend just takes it up another notch.