Another vote for taking them to the graveyard at midnight. Be sure to take along a digital camera with a view screen on it, because as you walk around you will kick dust and whatnot up into the air which can make for some interesting orb pictures on the camera view screen, which you can immediately show the girls and tell them that they are ghosts.
Also, small insects, like mosquitoes will show up green in the picture at night, if they are real close to the cameral lense, which makes for an interesting looking picture.
Yeah, I got bored one night and had a digital camera to play with.