Ercolano is closer to NAS Naples, but, depending on your choice of trains, can take just as long to reach as Pompeii. Be sure to check the route your train takes to Napoli Centrale and only take the Circumvesuviana train headed to Sorrento/Pompei (not all trains from platform 3 do).
Once you arrive at the Ercolano Scavi train station, head downhill for 400 meters. You'll bottom out at the psuedo-entrance to the excavation. From here you get a good aerial view of the site, which, after seeing Pompeii, seems very small. But Ercolano is better preserved than Pompeii; many buildings have second floors, doors, staircases, frescoes and mosaics still intact. As with Pompeii, many areas are being excavated and restored and therefore barred from view, but you still feel - here especially - like you've stepped back in time.
You also witness some of the destruction caused by Vesuvius's eruption: charred stairs, fallen floors, and rippling tile. You see how the ground moved, just like a wave in the sea. Mother Nature is frighteningly powerful, but beautiful too...
We ate a late lunch at the Luna Caprese Pizzeria on our way back to the train station (midway up the hill, on the left). This small and friendly eatery offers a variety of delicious wood-fired pizzas (size small in the US) for 3-4 Euros each. We split two Margherita pizzas, Shawn and I shared a small carafe of house wine, and Cole had milk all for less than 20 US dollars. It was so much nicer than the Pompeii Cafeteria, and we finally had our first authentic Neopolitan pizza!
I want to visit the Donadio Cameo Factory, something we didn't have time for on this trip, so we'll be returning to Ercolano again as well.
Total travel time: 3 hours
Time on excavation site: 2 hours
Cost: 50 Euros (transportation, entrance fees, and lunch)