Hello,
My husband and I will be traveling in October from Siena to Sorrento on Day 6 of our Itinerary. I have concluded from other posts that our options are:
1) bus from Siena to Rome, fast train to Naples, then Circumvesuviana train to Sorrento, OR
2) rent a car in Siena and drive to Rome or Naples or Sorrento. If we drop the car off in Rome or Naples we would still have to do 1 or 2 trains but at least it would be earlier in the day. If we go with the car option, is there a car rental outside the city of Siena?
The car seems like the better option because the first Sena Baltour bus from Siena to Rome doesn't arrive until 16:30. Then we would have to take the 17:53 fast train to Naples and then the 19:13 Circumvesuviana train to Sorrento. That is a long day with a lot of transportation changes. Wouldn't a rental car make more since, especially because we can leave earlier and make a scenic stop along the way? So many fodorites recommend against driving in Rome and Naples, but can't we just stay on the freeway/autostrada and go around Rome and Naples to get to Sorrento?
Thanks for any and all advice!

A little different but we kept our rental that we used in Umbria and dropped it off at Naples airport as we were staying in the city. The drive on the autostrada was easier than using trains and buses, dragging luggage on and off. I am sure you could find a way to drop it off in Sorrento too.Check pick up and drop off locations on AutoEurope.com. It was actually cheaper to drop the car off in Naples than at FCO where we had picked it up.

It's not clear... Will you already have a car or would you be renting a car just for this day?

If you already have the car, you could likely drop the car in Sorrento. Several rental companies have offices there. There could be a one-way surcharge.

If you'd be renting a car just for the day, you might want to get an estimate for a single day, one-way rental before deciding. You wouldn't be able to get a really early start because you'd have to wait for the rental office to open and then go through the rental process which can take some time if there's a line. It's a fairly boring 5.5-6 hour drive unless you plan an interesting stop or two. If you do plan to make stops, you should rent a car large enough to hold all of your luggage in the trunk, out of sight, and not necessarily the smallest, cheapest car.

A third option you didn't mention is taking a train from Siena to Naples and then train or taxi to Sorrento. Some options from Siena involve a connection in Florence which sounds like the wrong direction except that you could then take a fast train from Florence to Naples, and you could travel from Siena to Naples in about 4 hours.

Been there, done that. Actually, we rented a car in Florence, spent a week near Siena, then drove to Sorrento. We kept the car for the first day in Sorrento (hotel had free parking) and turned it in the second day, downtown. It made for a long day of driving (I think it was 6 hrs?), but we much preferred sticking with one form of transport and keeping our own schedule. Mostly stayed on the A1. Traffic as you get close to Sorrento can be congested on weekends, so if you have the option choose a weekday.

>>> Wouldn't a rental car make more since, especially because we can leave earlier
This restriction is totally due to the particular choice of the bus. You can leave much earlier by taking bus or train to Florence then 3 hr Freccia train to Napoli.
>>>make a scenic stop along the way
If you have time. Do serious math if you actually have time for detours. Many scenic stops are quite ways off the autostrada. I have driven this Autostrada all the way to Salerno. I couldn't remember any place along the way I would take this autostrada because of the scenery. Most towns worth stopping have ZTL preventing you from driving into town, adding more time to get in and out. Orvieto is one place close to the autostrada, but it has ZTL. The scenic option is through Val d'Orcia with option to stop by at many towns in the region. But I don't think you have time to do this.

It seems you are renting a car just for a day to do a part of this segment? Have you computed in time you have to wait until the office opens, in queue and paperwork before you can drive off, lunch/gas/rest stops, and car return overhead time. I think you need to factor in 2-3 hours in addition to the drive time to get the estimated total travel time. Even if the pure driving time is shorter than the train travel time, consider that if you are on the train, you can eat and use restrooms while the train is in motion.

Thank you for all the advice! To clarify - We are spending the first four days of our itinerary in Florence. So it seems that renting a car in Florence just before we leave and then use the car to visit the other little villages near Siena while we are there would make more sense. We can then leave early enough from Siena to make the 5-6 hour drive to Sorrento to return the car. Are there rest stops along the autostrada?

Yes there are rest stops and easy off and ons. They have restaurants, shops, petrol, etc.

Google "autogrill italy A1"

Does your Siena hotel have parking?