We'd planned on going to Castel Sant'Angelo but discovered it was closed on Mondays. There was an exhibition of highlights from the different regions of Italy so we spent some time trying to understand what was going on - all the descriptions were in Italian. Then we headed towards the Vatican and into Piazza San Pietro following the crowds. The queue into St Peter's was already stretched through the herding pens and across the Piazza, but we'd decided before we travelled that we weren't going in there.
I cannot recommend enough pre-booking your tickets to the Vatican museums. We'd booked our slot at 2pm - expecting to have been busy for the morning. All we had was an email so a little nervous we decided to figure out what we needed to do when our timeslot came. As we walked around the walls of the Vatican we quickly came across the other huge queue. Worried that we might need to wait in that queue we kept walking to the front and finally spotted someone who looked like they were in charge. He waved us towards the doors where we noticed a separate entrance. We went through the second door, passed our belongings through security and quickly got our tickets validated. It wasn't long after 12, which had been the first available slot we could have booked. So we thought we would head straight to the Sistine chapel in the hope that it might be a little quiet. HA!
The flow through the Vatican museums on the way to the Sistine chapel seems to be one way. However there were some diversions on the way to see other collections. We went for a couple of these diversions however soon learned that they were big loops that took you almost back to the beginning so you had to go through the same bit again. 
We learned as with lots of places that you have to time things just right. You need to get yourself between two tour groups. This doesn't give you a lot of time to stop, look and take pictures because you'll be caught and then have to wait for that group to pass you (and you'll end up with photos of their flags). We also learned to look instead of taking pictures - we still have a lot of photos. Several photos named Vatican floor 1, Vatican floor 2 etc. Vatican ceiling 1, Vatican ceiling 2 etc. 
By this time we were starving so found somewhere outside to sit and eat the snacks we had with us. After looking at the various versions of the Pope mobile and seeing some of the other exhibitions we decided we'd had our fill of the Vatican and the crowds.
We wandered aimlessly slightly dazed by the early start and the dramatic increase in heat. Italy is warmer than Scotland – who’d have thought it. We came across a little park outside the Quirinal and studied the map. Then we wandered on through tiny meandering street coming across beautiful Piazza’s, screaming mopeds and fashionable people. Almost by accident we stumbled across the sound of running water and rounded a corner to find the Trevi fountain. How can the street leading to the fountain be so quiet and yet the Piazza itself hold so many people? We took photos and watched people throw their coins. There was a champagne cork floating in the water, I wondered if someone had recently gotten engaged. 
We kept going and found the Pantheon – most of this time I thought we were just wandering turns out that O was wandering in a deliberate manner. Outside the Pantheon we bought a sandwich and discovered just how expensive things are in Rome. We sat on a wall and ate reading what the guide book told us about the Pantheon before we headed inside to look. I tried and failed on several occasions to take a picture of the roof and the Oculus. The guide books say that the Pantheon is particularly spectacular in the rain or the snow where the elements come into this ancient building unashamedly. Conveniently for us it started to rain while we were inside to demonstrate this wonder! So we spent a little more time inside than we expected.
Deciding that it might be time to go somewhere else to avoid the rain we took advantage of a break in the showers to head towards our hotel. After a nap and the rain had stopped, we went out to enjoy the rest of the day. We headed towards the Spanish steps – another must see tourist attraction, via the Piazza Navona. It isn’t exactly a peaceful city Rome and it continued to amaze me that so many people were crammed into all of these places. Thinking of the films that have been shot in Rome how did they ever get rid of all the people? We took pictures from the bottom of the Spanish steps, we climbed the steps and took pictures from the top.