One of the main basilicas in Rome is that of St. Paul outside the walls (San Paolo fuori le mura). If you are going to that basilica, obviously the basilica with its splendid and majestic facade and forecourt is one of the key points of focus for many, along with the the tomb of St. Paul and the high altar inside. However if you are going to that basilica, you'll also want to make sure to add a ticket to the medieval cloister which also houses a small but worthwhile treasury museum which includes a small sampling of the basilicas vestments as well as some of the church plate in the form of thuribles, chalices, ciboria and so on.
That museum is located just off the medieval cloister, which itself is an absolute must to stop and linger within, complete as it is with walls decorated with both Roman as well as Christian spoila that have been excavated on the site, as well as beautiful cosmatesque work done on the columns of the cloister.
