Most attractions in Rome are within walking distance of each other. You can also take the metro or bus to save time. A single ticket costs €1.50, and a 24-hour pass costs €7.
If you have more time, consider visiting the Pantheon and the Trevi Fountain. If you have less time, skip the Roman Forum and spend more time at the Colosseum. Remember to dress appropriately for the Vatican, covering your shoulders and knees. And, of course, bring comfortable walking shoes!