*doesn't have 3D glasses lying about*
What I've always thought would be interesting would be to have two full-color copies of an image rendered from two slightly different positions (moving the camera, like, a few centimeters to the side) beside each other, so that you can cross your eyes and match the images up on top of each other to get the 3D effect. That way everyone can see the 3Dness properly without having special glasses. Alternatively, you can set the two images as two quickly-looping frames of a GIF, which also gives the illusion of 3D, although of course that way you also lose some color detail.