Hello from Montreal, Canada
Rome is my favourite city!! Unfortunately, you won't be able to see the best in 2 days. Arrive early at St-Peter's or the Sistine Chapel (8am), you won't be able to do both in the same day (the crowds are awful and we could note make reservations for either when we where there last October for my b-day). Rome is a walking city. I hope your hotel is central. We stayed in Piazza Navona and walked everywhere. The hop on /hop off bus is a good way to see the city. Just walk everywhere and enjoy, go throw your penny in the Trevi fountain so you'll be sure to be back!
Enjoy ROMA as much as we did.