What is the Halacha about the Italian churches?

I’m most likely visiting Italy in the summer, and while I’m there I’m I plan on going to the Vatican, and a bunch of museums when I’m inside Rome, so my question is, as many of the museums are used in old church buildings, such as the ones in the Vatican, and many other ones in Italy, is it a issue to go in a church if it is not a place of prayer currently, but was in the past? What about shrines to the old Roman gods, are those allowed to be visited or is that Avoda Zara?