CitySightseeing Napoli (tel. 081-5517279; www.napoli.city-sightseeing.it) offers hop-on-hop-off double-decker bus tours of the city. Buses depart from Piazza Municipio/Parco Castello and run with varying frequency to all major attractions. Tickets are valid for 24 hours on all lines and can be purchased onboard for 22€ for adults, 11€ for children ages 6 to 15; and 66€ for a family (2 adults and 3 children 18 and under). You'll get a 10% discount with your Artecard.
MAN-Museo Aperto Napoli (tel. 081-5636062; www.museoapertonapoli.it) is the cultural organization serving Naples's historic district. It is based in a large cultural and information center (Via Pietro Colletta 85, to the right of Castel Capuano, at the eastern edge of the historic district; daily 10am-6pm) where you'll find a knowledgeable and kind multilingual staff, a multimedia center with information on the historic district, exhibits on the area, as well as a cafe offering typical local products, a bookshop, a small space with crafts for sale, and, luckily, bathrooms. The center offers all kinds of services useful to the tourist, from maps and information on the historic district, to umbrellas on loan and excellent guided tours: Choose among four self-guided audio tours or walking tours with a live guide; all tours are offered in six languages. At presstime, the service had been suspended temporarily with no deifinate date for re-opening. Ask at the tourist office for information.
Napoli Sotterranea (tel. 081-296944; www.lanapolisotterranea.it), or "Naples Underground," organizes guided tours of the city's abandoned aqueduct tunnels. The tunnels date back some 5,000 years and feature ancient Greek and Roman remains -- in particular a recently excavated Greek theater -- as well as more modern sites, such as the galleries used as hideouts during World War II. The tour lasts about 2 hours and meets at Piazza San Gaetano 68, on Via dei Tribunali near the church of San Lorenzo (Metro: Dante). Especially popular among older children and teens, the tours last around 1 hour and 20 minutes and cost 9.30€ (10% off with Artecard), 6€ for children under 10. Tours in English are scheduled daily, year-round, at 10am, noon, and 2 and 4pm.
For more classic city tours, the agency Every Tours, Via Santa Brigida (tel. 081-5518564) organizes tours of the city as well as day excursions to Vesuvio and other sights. It's open Monday through Friday from 9am to 1:30pm and 3:30 to 7pm and Saturday from 9am to 1pm. Another good agency is NapoliVision (tel. 081-5595130; www.napolivision.it), offering guided tours of Naples, Pompei, and Capri starting at 30€ per person (20% discount with Artecard).