I was just in Naples for several days and decided to spend some time in Caserta, a 45 minute direct train ride away. The main site there is the Reggia di Caserta, a huge Baroque palace which is actually the largest palace in all Europe. It is a five minute walk from the train station, so it works well as a day excursion from Naples. The palace itself is amazing, 1200 rooms, but mercifully you can only tour 50 or so of them. The EU has poured lots of money into restoration, so the rooms open to the public are beautifully redone. And the gardens with incredible statuary along the 2 mile cascading pools, ending with a waterfall, will take your breath away. There is a shuttle for 2.5 euros that takes you to the base of the waterfall, and you can return on the same ticket. Personally, I prefer to walk back for better views of the fountains and palace. It is slightly downhill on the way back.
There is an English language audioguide that you can skip unless you really want to know who designed each room and which Bourbon royalty stayed there. There are signs in almost all the rooms with plenty of information inItalian and English, so I stopped playing my audioguide after a while and just depended on the signage.
This is a UNESCO World Heritage site, and it amazingly is not crowded at all! The town of Caserta is pleasant enough, but not much to see other than the Reggia. Hotels cost about 50 percent of similar hotels in Naples, and the restaurants are also less expensive. Also, it feels much safer walking around there. As a single woman traveler, I felt quite comfortable walking back to my hotel at night. Since I stayed two nights there, I took a local bus to the hillside town of Casertavecchia, a medieval town with twisted streets, old stone buildings, snd no American tourists at all.
Naples can be intense, and the Amalfi coast, while breattaking, is overrun with tourists. A daytrip or even overnight to Caserta would be a good addition to a Naples itinerary.