Inside St Peters
One more from vatican city. If in Rome I highly reccommend to visit Basilica of St Peter. Unfortunately I was not able to visit any other sites, although I would have liked too! If you plan on visiting make sure to arrive as early as possible to avoid the queues.
It is beautiful. I found the crowds there overwhelming though. When I visited it was the end of tourist season and it was so packed. Still, definitely something you must see if you're in Rome. The decadence is substantial.