Hi Shelly.
The weather can be unpredictable in October. My husband and I were rained out in October 2013 ditching half of what we originally planned. We returned the same period in October 2014, and there was no rain until the very last day. A lot of walking involved. It is warm and a bit humid, too. We purposely chose Sorrento last year for all-weather conditions(access to train, bus, ferry, and car as well).
As for sightseeing, Circumvesuviana train covers the most. Yes, Pompeii is a must for first timers, but I found Herculaneum more fascinating. I appreciated there more because I have previously visited Pompeii a few years back. Oplontis(Villa Poppeia) is beautiful. It is a bit far from a train station, but villa San Marco in Stabiae has been our most favorite ruins. And you will have a whole new respect if you climb to the summit of Monte Vesuvio.
From Napoli, it's so easy to get to Sorrento by taking a ferry...straight shot to Sorrento in 35 minutes or so instead of taking a train for 70 minutes.
We are returning to Napoli this October as well. There is always "unfinished business" to cover, and we will be visiting Pozzuoli, Baia, Bacoli, etc.
Happy Planning!