Do this!
The pass gives you metro travel and entry into a couple of sites of your choice (means you do not have to queue in the same line as everyone to buy ticket), I suggest itinerary as follows...
Day 1:
- Buy Roma pass at central train station
- Use pass to travel from station to the Colosseum metro station - this will be the ONLY time you will use the metro as Rome is tiny.
- Colesseum, skip the ticket line as you have Roma pass and there is a different queue...nifty. Hum Gladiator sound track and shout "single column" with appropriate arm movements.
- Forum - use the pass again at the Forum which is right next to the Colesseum. This is a large site and takes some navigating to make sense of it. You'll literally find yourself walking over Roman mosiac flooring thinking "is this legal".
- Once finished at the Forum walk towards what I would call Rome's centre make a b-line for the Trevi fountain. It is just a fountain on a back street and takes no time to see.
- Walk to the Spanish steps (TBH this is nothing special at all, I would say skip this)
Day 2:
- Vatican city and Cistine Chapel and St Peters....basically WOW on these. You'll need 0.5 to 0.75 days to do these.
- Head for the Pantheon - stopping off at Piazzo Navona for an ice cream and look at the fountains.
- Pantheon - must see. Amazing building.
Day 3:
- You'll have a good feel for Rome now, so take a little more leisurely pace and have a walk to some of the many churches which are very impressive in themselves.
- Go to the Museum - you'll see many sculptures (ancient Greek ones - such as one of Apollo - that is the earliest sculpture in EXISTENCE that depicts movement).