We visited Sorrento as part of an all-day tour that also included Pompeii. You can easily spend a couple of hours walking around its streets. The main area is only about 16 blocks, if I recall correctly. There are a couple of places where you can purchase inlaid wood items -- I purchased a beautiful box for my bureau top that didn't cost much. We also enjoyed a walk down to the bay. There are also a lot of shops that sell the basic tourist items -- we skipped those and went straight for the gelato!
Is this the tour that also includes a visit to the family-run farm for lunch?
Also, I agree with Hank about Herculanium and Naples. We spoke with several passengers on our cruise who had visited one or both, and we made a mental note for ourselves to consider these next time.