Ciao Ed!
I live in Rome and we travel often south tho Sorrento, Naples, and Capri. Check out this posting for our most recent trip to Sorrento:
Getaway to SorrentoThere are plenty of car parks available near the port in Naples. I recommend Naples only because there are more options than Sorrento and we've used the car parks there before. I would Google "Car Parks" in either Sorrento and Naples and you'll get many hits - probably in Italian... like these folks: [url=http://www.carsudpark.com/] .
If you're already locked into your hotel on Capri, so be it... but I'd rather not go back and forth on the hydrafoil unless I had to. Once we get to Capri, we just tend to stay there - It's very quiet and relaxing. Naples is exactly the OPPOSITE! But if you have limited time I understand your dliemna. If you could, I might change to 2 -3 nights in Capri and then 2 nights in Sorrento... you get the best of both worlds. Oftentimes when we stayed at a hotel in Naples or Sorrento, we coordinated with them to LEAVE our rental car and pay a daily fee. This is another option to consider if you're spending a night before heading out to Capri. Good Luck - you'll have a great time in Capri!
Ron
Ron-in-Rome