We did Pompeii on our own after the Viva Italia trip in June 2007. It took most of the day. We went by train and left Rome around 7:30 AM. It takes approximately 1:30 to get to Naples. The only tricky part is changing trains in Naples to a local commuter line to Pompeii. The tour books warn about pickpockets on this part of the train, and while we didn't have any problems, it was a little unsettling. However, a local man told us we were waiting for the wrong train and politely showed us which train to take. We arrived in Pompeii around 9:40 AM. The train cost approximately $69.00/pp each way.
We thouroughly enjoyed Pompeii and spent most of the day there. We purchased the audio headset tour. You do it on your own and it takes several hours, but so much more informative. Without the explaination, it all looks the same after a while. You can tailor your tour based on your interest. I was surprised how big Pompeii was. There's a cafeteria where you can get lunch. It's very dry and hot. Plan for a few water breaks.
We took the 4:40 PM train back to Naples and arrived in Rome around 7:30 PM. We were exhausted. Pack comfortable shoes. The cobblestone streets are more difficult to walk on than we expected.
All in all, it was one of the highlights of our trip after Viva Italia and glad we did it. If you can work it into your schedule, I would highly recommend a day trip to Pompeii.
A suggestion for a shorter side trip...we toured the Borghese Gallery on the day we arrived. After
ABD checked us in, we unpacked and were off. To save time, we took a taxi and walked back to the hotel afterwards. On our Viva Italia trip, we stayed at the Hotel Excedra, which is about a 5-10 minute taxi ride from the Gallery. If interested, you have to make reservations in advance. We did it online and had no problems. Again it was one of the highlights of our time on our own. It's a small gallery, but some of the most amazing sculptures we saw.
Bookending the Borghese Gallery at the start of our trip and Pompeii at the end, we felt like it made for a memorable beginning and end to a perfect trip.