I did an awesome one several years ago when I worked for The PocketPhone Shop. We were in a conference hall, thinking we were there for a big annual meeting and after a while a group of 3 blokes in long black coats and sunglasses walked in and 'took over'. Shortly after that the big curtain that divided the conference room came down and beyond it was a HUGE warehouse that was laid out with loads of Computer terminals. It was done on a 'Matrix' theme and there was a whole days activities with all kinds of stuff like where you are in a maze of corridors with the laser guns and computer based puzzles etc. I haven't seen The Matrix, so I don't know the storyline, but it ended with the large cube in the middle of the room exploding with an absolutely deafening bang.

Brilliant day.