The Piazza Navona, Pantheon, and Trevi Founter are close together and it's easy to walk from site to site.
I added a map from Google below to give you an idea of the distance between sites. The Spanish Steps are a little farther away but if you have time, you could walk to it while you are in the area. We didn't have time to see it since we had to catch a plane
Since we were so short on time, we took a taxi, instead of the bus, to the Piazza Navona. I have seen this piazza is so many movies so it was on my to-see list while we were in Rome. It is also a great starting point for visiting the Pantheon and Trevi Fountain since they are just a short walk from there.