I'll vote for either taking a ferry from Naples to Capri and back, or just staying in Naples. Depends on the weather as the island wouldn't be a lot of fun in the rain. Someone mentioned Herculaneum but you could just as easily do Pompeii: abt. 1/2 hour by the same cheap commuter train from Naples you'd have used for Sorrento. Again, could depend on the weather as it's mostly an outdoor site, and it's huge. Opens at 9:00 AM.
https://www.sorrentoinsider.com/en/naples-to-sorrento-train-schedule
If you did Pompeii I would make sure to get back on the Circumvesuviana in plenty of time to get back to Naples. Same with a ferry from Capri. Whatever the case, your original plan isn't do-able for reasons already listed but not to worry, you'll still have interesting things to do! Yep, the Archeological Museum in Naples is where a lot of the treasures from Herculaneum and Pompeii reside, and gets high marks from posters who've been there. It would be a good choice on a rainy day.
https://mann-napoli.it/en/home-english/