Santa Pudenziana is one of Rome's oldest churches, tucked near Santa Maria Maggiore and easy to overlook. The reason to seek it out is the 4th-century apse mosaic, one of the city's great early Christian works, set above a small interior that only needs a short visit.
Opening hours can be limited, so it is worth checking before you go or trying in the late morning. If the doors are open, fifteen quiet minutes inside is enough to make the detour pay off.