The weather in Naples is lovely year round. I usually go there at the end of March. The days are in the upper 70s, low 80s, and the evenings are cooler. Aside from shopping, I am assuming that you will be spending a lot of time on the beach. So, of course, beach wear is most appropriate with some lovely beach cover-ups. In the evenings, I usually dress warmly. I freeze in air conditioning, and even the warm evening air seems nippy after sitting in the sun. You can get by with nice jeans and wedges, or summer dresses and skirts. But take summer-weight sweaters. I would suggest looking at the resort looks coming out at Jcrew, nordstrom, banana, etc... to get ideas.
Oh, and Naples is a bit hoity-toity. I would suggest looking nice as can be at all times unless you want to attract the glares of the local ladies.
