I've had fun on a few days when I was working. Once I put an OUT OF ORDER sign on the men's loo - it was just near my desk and I could see the door by shifting my chair a bit- telling the fellas they'd have to go up or down a floor - and they all obediently went! Not one questioned it. That one was a hoot, my most successful.
Another year I went in early and changed the photocopier language to Japanese (carefully memorising the right symbols to turn it back. That was fun too!
