Not sure about the health and safety implications for use in the office environment, but this little radio controlled helicopter looks like a lot of fun:

http://www.radioshack.com/sm-2-channel-infrared-picco-z-mini-helicopter--pi-2431387.html

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Updated 2021 : note that the above post is out-of-date, given this post was originally published in 2006, but is left here for archival purposes. I have unlinked the now-broken external link.