We will be in Italy for about 2 weeks in late May. Will definitely spend several days in Rome, several days in and around Sorrento (Amalfi, Pompeii, Paestum, etc.) and a day in Naples. Debating what to to do with the other 4-5 days - we are considering driving north from Rome to Siena, Assisi, and Orvieto (Civita) OR driving east to Abruzzo area and/or Gargano Peninsula. We like to combine city/museum stuff with countryside/relaxing/outdoorsy stuff. We don't want to spend too much time driving (optimally, no stretches longer than 4 hours between major destinations). We are not really big on beach resorts where the main activity is sunbathing, but wouldn't mind one relaxed beach day. What general areas will be most interesting and "high-yield"? What are the activities and sights in Abruzzo or Gargano? I noted that the RS Southern Italy tour goes to Gargano and the area around Matera BUT these areas are completely excluded from his guidebooks. Thanks!
Since you mentioned outdoorsy stuff, I would consider Abruzzo. This area is conveniently between Rome and the AC, although inland. It is mountainous and beautiful with many small picturesque towns. It is certainly off the beaten path. I think it would give you a nice contrast to Rome and the AC. There is an agriturismo called Italy Farm Stay that is highly rec on this site. I have no first hand knowledge of the place but I am very familiar with the area as we have an old family home there. I know that there is horseback riding and parasailing and hiking in the area and many other activities. I also love Orvieto, Siena and Assisi but that involves more driving. I would save that area for your next trip.
The nice thing about Siena would be it's accessibility by train, but since your are driving, that probably won't be a factor. It's still a great smallish city with a wonderful Duomo and campo (simply Il Campo) and a fun history. It's also a great jumping off point for other small tuscan hill towns and wineries. I don't know much about the others, we wanted to check out Gargano and the Tremiti islands, but decided against it since it was a bit out of the way for us. I imagine much of the allure would be similar to that of the Amalfi coast or Cinque Terre, though.