Delos Island, Greece.
Access to Delos is only via Mykonos. A selection of boats leave every morning (weather permitting), except Mon, from 9am onwards from the old harbour of Mykonos (south mole) and make the 30 minute crossing to Delos, returning regularly up until 3pm when the site closes. Average cost: €12 r/t. Rhenia, which is also a restricted archaeological area, can only be visited by making private arrangements with a caïque service in Mykonos.
Delos Travel Guide