Hi Anne- advance congratulations on your 30th! Your experience might be different, but on our trip in December 2012, we had an apartment in Rome (Trastevere neighborhood - highly recommended) for a whole week, using buses, trams, and an occasional metro or taxi ride to get around. With the wealth of museums and ancient Roman sights, we needed a week to scratch the surface, and still left with a lot to see and do. After flying to Sicily and picking up a rental car, we drove it back onto the mainland, so for our 4 nights in Sorrento, we had a car. We actually were at a B&B just up the coast in Piano di Sorrento, so except for our arrival night, when there were pre-New Year's festivities and parking spots were hard to find, we didn't have any parking problems. Driving to Pompeii, Herculaneum (smaller than Pompeii, but at least as impressive for the structures and frescoes that survived the Vesuvius erruption), and other sights was convenient and easy (make sure you exit at the right place off the Autostrada). It would have been nice to have had more than 4 nights in the area, and that allowed a day trip to Capri but not an overnight stay. If there'd been more time, at least a night there would've been nice, and also the seas were too rough that day to get into the Blue Grotto, so weather can make that experience hit-and-miss.
We took a train to visit Naples, and its Archaeological Musuem was worth the trip (plus a chance to have authentic Neapolitan pizza), but it was pretty gritty and while others might devote a longer time in Naples, having just the one day was plenty for us.
Our B&B host said if there was one time of the year he'd visit Sorrento, for the best combination of pleasant weather, smallish crowds, and available activities and sights, it would be April. Spring will be warmer than December was, and way less crowded and hot than summer.