Hi Doug,
I've been to St. Peter's Basilica a number of times. I found it to be absolutely gorgeous and breath-taking. There is much to see in St. Peter's - I will leave it up to your vision when you visit.
As you enter, to your right, will be La Pieta. Sadly, it is behind some pleixglass due to some previous vandalism. If you are taking a picture or two, keep that in mind.
If inclined, there is also the Chapel of The Blessed Sacrament. It is a Chapel for prayer, reflection and silence. It too is gorgeous with its ornate altar and statues. I leave the rest for you to be in awe of.
For that Chapel, there is no picture-taking, no cell phones, no talking. Guards will tell you before you enter. And, there is a big white sign reflecting the same. I witnessed a couple of peeps who decided to gamble and abuse the rules. Guards do random checks - they were caught and escorted out of the Basilica.
A few things to keep in mind -
You will go through a security check - hats and jackets and backpacks/bags are to be removed.
No big backpacks are allowed - it was safe with around 14"-15." Big backpacks need to be checked (same rules as The Vatican.)
No food. Water bottles were allowed if put away in a bag or jacket.
No big umbrellas - checked. There is a "monitored" free bag check outside St. Peter's - down near the steps - there are also (free) restrooms.
If you or anyone in your party use a mobility device, they are allowed after a security check.
There are cute gift shops in and out of St. Peter's. If you want Holy Water, you can bring your own "little bottle" or buy a mini glass one in the gift shop (3€) You can fill it with a little bit of the Holy Water from Acquasantiere Holy Water Font - two big, beautiful, marble Cherubs adorn the font. When I was there, there was Holy Water in the Font. They seem to keep it filled.
Don't forget - Women and Men need to cover their shoulders and knees.
Enjoy St. Peter's and The Vatican.
