After You've Done St Peter's And The Colosseum...
St Peter's keyhole
Before you leave the hill walk down a block to the right and you may see people crowded around the entrance to the Sovereign Military Order of Malta. Why? If you look through the keyhole you get a perfectly framed St. Peter's. Keeper of the keys!
Ai tre scalini
This fun neighborhood is always guaranteed to be full on any day of the week, and has a great local vibe, though the bartender's speak english and the locals are more than willing to try their slurred English on you. Also probably one of the few places in town where you can grab a proper beer.
If you're still up, head to Trastavere, a nearby neighborhood with good bars and dancing. Avoid the bars with drink specials as most of those are filled with tourists and international students (unless that's what you want), and try to find one with the doors closed - that means they're not necessarily looking to be flooded with out of towners.