You don't have to be religious to appreciate the tranquil atmosphere of Cathedral of Santa Maria Maggiore. Check when the next service is scheduled or when you can go for a wander through.
You're spoiled for sightseeing choice when spending time in Santa Maria. Cathedral of Santa Maria Maggiore is a good place to begin here, but there are plenty of other worthy places to discover. Challenge of Barletta, Colossus of Barletta and Palazzo della Marra are near the principal church and will top off your day nicely.