I don't know about the Amalfi Coast or Positano, but it should be very easy to visit Pompeii on your own. Take the train, about a mile from the port, but walking to the train station is considered unsafe. Take a taxi to the train station. Take the Circumvesuviana Line to Pompeii Scavi & Ercoliano Scavi where the stops are close to the ruins. Buy a combination ticket, app 18 euros. This is admission to both Pompeii and Herculanium. Pompeii is big and you can spend a lot of time walking around.