When you first get to Naples take a Naples Trolley Tour. They can be picked up at about 25 different places and you can get on and off as you like.
This is a very good way to see all of Naples so that you can get a better idea of where to go when you're on your own.
*CAUTION* If they say they're at a stop at a certain time, and its the last stop of the day, be there EARLY!!!! Finding a taxi in Naples is not fun.
The Trolley meanders all over the town of Naples, giving you a 2 hour narrated tour of the area that is comprehensive & fun. It's a great way to see what's what & get oriented to the interesting locales.
Officially, the tour begins at 6th Avenue South and 10th Street South, but you can pick it up at any of the marked Trolley stops in town. Maps are available at the website listed below, and printed copies are available all over town (including at the Chamber of Commerce).
However, because it does a 2-hour circuitous route & costs about $16-20 pp for adults, it isn't really a viable means of transportation in general. Once you've bought a pass, it's good for the whole day, & you can get on & off as you please - just be sure you don't miss the last one leaving your stop to get back to the point of origin!