As a couple, give Mouth of Truth (Bocca della Verità) a miss.
A genuine two-minute photo stop and nothing more — you queue, put your hand in the marble face from Roman Holiday, snap, and go. The honest verdict: only worth it if you're already at the Bocca, because it sits in the portico of the lovely church of Santa Maria in Cosmedin and a short walk from the Circus Maximus and the Aventine. Cross the city for it and you'll feel cheated; fold it into an Aventine loop and it's a fun beat.