it's rare when we have more than one houseguest at a time, but it does happen from time to time. and on those occasions, the guest bathroom gets its fair share of traffic. i've thought about using a visual cue on the doorknob - a tie, a tassel - something to show that the room is occupied.
after our visit to maine, i was completely inspired to pick up one of these cabin door locks that my father-in-law/sailor-in-law had installed in one of his bathrooms.
yes, it's brass, which is unlike everything else in our guest bathroom, but i still dig its old timey, nautical feel. plus, this thing is solid and won't pop off if someone pushes hard on the door.