ferry  From the main ferry terminals in Naples Port "Molo Bevellaro" or Sorrento Port "Marina Piccola" (Small Port) you can take an ferry to the island of Capri, Iscia, down the coast to Positano and Amalfi or up to Naples (perhaps for the Museum?) all are relatively inexpensive, run regularly & are easy to use. 2009 prices high season are around 6-15 Euro depending on destination and speed of the ferry/boat.  Be sure and TRIPLE check the schedules while at the port, from your hotel, or just call them.  If your "home-basing" in Sorrento, then the ferry/boats are best to take in the morning before 9am so you get enough time at your destination before the last ferry/boat back.  Capri is the most populous route so they have multiple times and services to the Island and the last ferry/boat leaves around 7-8pm, so don't plan on an Italian style dinner in Capri if your boating it.
 Herculaneum  : A smaller but better preserved site than Pompeii, with less people & more of a complete feel, well worth a visit, reserve about 2-3 hours for best effect! Costs the same as Pompiee at 11 Euros per person or you can buy a combined 3 day pass to the suites for about 20 Euros per head. It is a few stations on from the Pompeii stop (at Ercolano) a few hundred yards down a steep hill near the sea, it can easily be combined with.............

 

Vesuvius: A trip from the Ercolano station takes about 20 mintues plus another 20 minutes to walk to the peak, entrance is about 6 euros with transfers between 8 & 10 euros per person. Allow about 2 hours in total, so you can combine this well with the Herculaneum.
 

Amalfi Coast tour: Via either coach, private (or hire) car or boat, you can take a gentle journey along the scenic Amalfi coast. 

Ferry/boat trips:

Private tours: All the above can be booked as either a private tour in your own air conditioned car, or boat charter, there are also plenty of group coach tours combining the above.