Every tourist who makes it to Rome certainly tries to get into St. Peter’s and the Vatican Museum. Naturally, many of them make it over to the Coliseum even if it is just in a taxi ride around the building.
I would recommend that anyone going to Rome for the first time make every effort to visit the excavations below St. Peter’s. These excavations are extraordinarily interesting. When Constantine wanted to build a church to honor Peter as the first bishop of Rome, he was able to identify the place where Peter had been buried after his martyrdom in the Year 66. The cemetery was on Vatican Hill just across the Tiber from the city. Constantine clipped off the top of that hill and filled in the lower portion creating a level area for the construction of an enormous church, and that church would last until the 16th century.
The church was approximately the same size as the present St. Peter’s. Filling in the area on the down side of the hill protected the mausoleums that had been constructed there centuries earlier. They would actually not be rediscovered until excavations in the 1950’s. Constantine built the main altar of his church over the exact spot where St. Peter had been buried. When the present St. Peter’s was constructed in the 16th century, the same spot was chosen. When you go down into the excavations, you come to the exact spot where Peter was buried following his martyrdom. It is a very moving experience.