7. The other Major basilicas (St. John Lateran, St. Mary Major, and St. Paul Outside the walls).
8. The Holy Stairs (near St. John Lateran) and the relics of the cross in the church of Santa Croce (also near St. John Lateran).
9. Catacombs of St. Callisto or Sebastian (both are nice).
10. Vatican Gardens (reservations here). If you get tickets to the Vatican Gardens, the ticket for the museums is included.
11. Borghese museums (reserve here months in advance).
12. The church of Tre Fontane, where St. Paul was martyred.
13. Mamertine prison, where St. Peter and St. Paul were once held prisoners.
14. Dome of St. Peter's Basilica - (6€ to climb all 551 steps. 8€ to take the elevator and skip 320 of those steps).