On our cruise in June 07 we did the RCCL excursion to Capri. It was nice and we saw a lot. Also saw enough to know that this was something that you could do on your own. The hydrafoils to Capri are a short walk from where the ships dock. On the Capri side, you are right in the center of things. Buses are available to take you to Anacapri, and the chair lift to the top of the mountain. The main 'town' is easy to walk.
Check out the ferry scedules and give yourself plenty of time to get back. They tend to be crowded.
Photos from our day are at Naples / Capri
There was quite a haze for over half the day. Nice nonetheless.