Mount Vesuvius, the volcano that hovers above the Bay of Naples, is part of the National Park of Vesuvius and is open to visitors. From the parking area and ticket office the walk to the summit of the crater is about 20 to 30 minutes uphill on a rocky trail. On the way up there's a good view of Naples. Mount Vesuvius Visitor Information gives you an illustrated look at visiting the volcano.
Transportation from Sorrento
From Sorrento, take the Circumvesuviana train to the Pompeii-Villa dei Misteri stop. From the station in Pompeii there is bus service by Busvia del Vesuvio. Check the train schedule on the Circumvesuviana web site.
Some hotels in Sorrento and on the Amalfi Coast offer excursions to Mount Vesuvius, probably the most convenient way to get there.