I did the following: printed some funny pictures from the internet, stick them in out company's interoffice envelopes, put the names of people addressed to (no sender name!) and distributed around by walking into offices and saying something like "hey, I came across an envelop with your name on it, check it out, might be important, may be from HR or something" and leaving immediately and wait for reaction! Pics of cute puppies for dog's owners; picture of adorable kid pulling plug from computer for females, funny computer and car pictures for guys.
Some were thinking hard who may send these; accounting girls checking with buyers; looking into faces of people passing by, comparing to a picture of 3 y.o. kid, this sort of thing!
Tell you, although mail tricks are simple classic, it generated some laughs, and that was worth it.
Did you do something?