We used the Rick Steves-Partnered company, sharedtours.com -- for our day trip from Sorrento to Amalfi and we were pleased. The scenery and drive along the coast is stunning. While it's a fast day and not a lot of time at each stop, we did find it to be enough time. We loved the area and are glad we went, but we really loved our 4 days in Sorrento also.
And just FYI, we also used a private driver from the Naples train station to Sorrento and from Sorrento back to Naples train station and we wish we would have used the regional train instead - the TRAFFIC was terrible. Esp going back from Sorrento to the Naples Train Station - it took almost 3 hours - we missed our train from Naples to Florence. The driver told us work-week mornings are always terrible traffic from Sorrento to Naples, - so if you do use a private driver, def plan for LOTS of time.