The former prison island of Santo Stefano is the classic excursion from Ventotene. Tiny, 27-hectare (67-acre) Santo Stefano lies just 1 nautical mile east of Ventotene. There are many former prison islands in this guide, but what makes Santo Stefano a standout among them is the unique architecture of the penitentiary, built in 1795 under the Bourbon Ferdinand IV, king of Naples. From above, the entire structure looks uncannily like a handcuff. The prison architect, Francesco Carpi, designed the cellblock buildings (which are rather attractive, with three stories of arcaded loggias painted sunny yellow) in a horseshoe shape; the cells face an inner courtyard where the guards' chapel-like watchtower had a 360-degree view of the inmates. Carpi was following prison design theory laid out by English philosopher and legal reformist Jeremy Bentham, who postulated that the panopticon ("all-seeing," round and inward-facing) prison not only made for easy supervision but also that the very nature of the architecture exacted a psychological toll on the prisoners -- "a new mode of obtaining power of mind over mind," as he described it. A very deliberate feature of Carpi's architecture is that none of the cells has even a sliver of a sea view. The prison was closed in 1965, but during World War II it was used as a holding facility for such outspoken anti-Fascist "hindrances" as socialist Sandro Pertini, who went on to become president of the Italian Republic (1978-85). The prison is technically private property, not a museum or monument (no admission fee, no contact information), but visitors can get as far as the inner courtyard of the cellblocks.
The other must-do on Santo Stefano requires a bathing suit, so be sure you've brought one: At water's edge on the southeast part of the island, below the ruins of a Roman villa, is the Vasca Giulia (Julia's Tub), one of the most memorable places in the Pontine archipelago for a swim. This circular pool was hewn into the smooth basalt in the 1st century B.C. and connected to the sea by means of curved canals in the rock. When the surf comes into the cove, the force of the waves is channeled into those narrow passageways, creating jets of water and a sort of Jacuzzi effect inside the tub. Roman hydro-engineering never ceases to amaze!
There are no regular ferries to Santo Stefano, but it's simple to arrange sea transport there and back with any of the boatmen down at the Porto Romano on Ventotene. They'll typically charge 10€ to the island and back, or a few euros more if you want them to make a stop at the Vasca Giulia.