I will be in Rome with my family on a Rick Steves tour that will include the Colosseum, the Forum, the Pantheon, the Vatican museum, the Sistine Chapel, and St. Peter's Basilica. We will stay in Rome one additional day. We would like to tour the Borghese gallery and would like to get suggestions of the most important sites to visit on our last day.
If the tour doesn't include the Pantheon, Piazza Navona, Campo de Fiori, and Trastevere, those would more than fill a day. The Borghese is wonderful, too. My one experience with a tour is that you might be so exhausted, sitting at a cafe in Piazza Navona or one of the areas I mention might be a great way to spend a few hours. I enjoyed Rome much more the second time around when I could slow down, so taking it a little slower your last day might really add to your enjoyment of this wonderful city.
The Galleria Borghese would make a great afternoon stop. You will be limited to two hours in the galleries, plus you need to be there 30 minutes before your appointment time, so it's manageable to do that and hang out in the morning. After the Borghese visit you can walk through the extensive grounds, stop at the Pincio, and take the footpath down to Piazza del Popolo.