Emily--
We'll be there not long after you, also staying in Sorrento. I think what you have to do, given that it won't be high season and fortunately you won't have to deal with making reservations or with crowds of tourists, that you should have a list of things to do and upon arrival figure it out on the fly based on the weather.
As you've no doubt researched, temps should range in the 40's and 50's, not bad. But if it's raining hard, you probably don't want to do Capri, Pompeii or Ercolano on that specific day. A light mist or on & off, grab your ponchos & umbrellas and go. Hard to wait that kind of thing out if you only have limited time. But there are also buses to the other side of the peninsula--the Amalfi Coast, train or bus to Naples, and then bus to Salerno and then train to Paestum.
But keep in mind that the A.C will be pretty much buttoned up for the season in mid-February as far as choice of restaurants, hotels, etc. Two years ago in early March we took the bus from Salerno to Amalfi-town, and the restaurant we lunched at on the water was their first day open for the season. The good thing, however, is that without traffic or tourists it was all the better to view the magnificent scenery. Definitely worth a trip.
Couple things--ferry to Capri will be hit & miss that time of year due to the weather. And if you haven't thought about transport to Napoli Capodichino airport, check out the Curreri bus--10 Euro per person direct from Sorrento station to the airport, takes 90 minutes. http://www.curreriviaggi.it/