If you happen to be Catholic, and you want to attend Mass, be sure to inquire about Mass times at the church. We have been able to attend Mass AND tour the church when it is closed to the public at St. Vitus Cathedral in Prague, St. Mark's in Venice, and the Sagrada Familia in Barcelona. We have also attended memorable Masses at scheduled times in many other cities, such as Notre Dame in Paris.
Non-Catholics are free to attend mass. No ID required.
I recommend attending a church service from time to time if you're sincerely interested. When visiting Burnham on Sea in England, my Episcopalian partner decided out of curiosity to go to the morning service at the local Anglican church. It was a weekday, and we were the only people there other than the priest and the church secretary. We were treated royally and got the full tour including the baptismal font which was said to date back to Norman times. The tour included obscure architectural details in a manner sweetly reminiscent of the visit of the imposter Bishop of Matabeleland in the movie Kind Hearts and Coronets (minus the poisoning, of course). When our B&B hosts learned of our plans, they got up earlier than usual to serve us breakfast in time for us to be on time to the relatively early service. It was an entirely charming morning.