The Amphitheatrum Flavium to The Forum to The Pantheon to The Trevi Fountain to The Spanish Steps is a nice transition route in a morning to evening run. The Vatican is also interesting in the evening. Consider a Roma Pass.

...Suggestion for Rome: Spend time out at night. If you're only going to be there for ~3 days, don't get too worn out looking at the ruins to enjoy the really beautiful side of Rome. The Spanish Steps and the Trevi Fountain are well within walking distance of each other, and are both spectacular after dark...