Run around in a white sheet so they'll think they caught a real ghost on camera!
Call up their rooms and make them believe that they're receiving calls from the dead.
Build your own ghost hunting devices that you can secretly control so you can scream, "OMG, I'M GETTING A STRONG READING HERE!"
Sneak a bunch of plutonium in and smear it all over everything so that their Geiger counters go nuts and render their ghostly readings useless. Because using a Geiger counter to hunt for ghosts is usually so accurate.
Bake ghost cookies and pie.