Although it borders on being a Helpful Household Hint in the tradition of Heloise, I thought this trick was neat enough to publish here. For apartment dwellers, particularly in student barrios where there's a large pedestrian population, having a spring clip mounted near the door can be very handy. It provides a convenient place for friends and other visitors to leave notes, in case they come by when nobody's home. Although it's easy enough, generally, to mount a clip by drilling a small hole in the wooden door frame and securing the clip with a screw, this procedure requires access to at least a screwdriver and probably a drill, besides putting a hole in the landlord's door. If you want the installation to be vandal-proof, it requires purchasing a special one-way screw that cannot be removed once screwed in.
My happy discovery is that the loop at the end of one of the wire arms of a standard medium-size ACCO office clip fits neatly around the shoulder of a standard door-peephole casing, but is effectively contained by the flange at the objective end. By unscrewing the peephole using a penny from the inner side of the door (see below), it is a simple matter to slip the peephole casing through one end of the office clip and then reattach the peephole to the door. The resulting installation requires no tools other than a penny, leaves no permanent marks, and is easily removed (but only from the inside of the door).