If I'm ever flying with non pilot friends I always brief them that they are an extra pair of eyes. People enjoy being given that bit of responsibility and I find that because your pax aren't distracted by anything else they tend to spot traffic before you. Always praise them if they do and give them a sweet.
The ones I hate the most are 'A/c on reciprocal heading. no height information' and you can't spot it, is he below me climbing, is he above me descending... always a worrier for me that one and an opener for interesting manouvres as I try to cover the blind spots.