Did you know that it's possible to climb up to the top of St Peter’s dome? It's quite a climb and I do not recomend it if you're claustrofobic or afarid of hights.
As always, if you intend to clomb to the top, go in the morning, before most of the tourists are awake. They open at 8. Or if you want a view with a sunset, so just before closing time. I reccomend that you buy you ticket before hand online, just to avoid the ticketline. During high season the line will eat up half your day.
At the top you will see the view of St Peter’s square, the Sistine chapel, the Vatican museums and the the Vatican gardens. Amazing right?
It's possible to take an elevator, for like 300 steps. That will cost you about 10€, 8€ if only take the stairs. I would redomend that you take the elevator up, but do the walk down.
The dome it self is a piece of art, but that shouldnt come as a surprise as Michelangelo himself designed it. Dont forget to check it out, there's much to see on the way to the viewpoint.
And dont forget, St. Peter’s has a strict dress code: no shorts, no skirts above the knee, and no bare shoulders. They will not let you in if you're not properly dressed. And you are not allowed to bring in large bags, but they do have a bagage drop for free.