Thassos is the closest Greek island to the mainland coast and one of the most rewarding for a day trip from Kavala. Ferries run from the port year-round; the crossing to Limenas (the island's main town, also called Thassos Town) takes about an hour. The island is green and hilly, well-forested with pine and plane trees, and has better beaches than anywhere on the nearby mainland coast.
The ancient remains at Limenas are exceptional: a substantial agora, a theatre carved into the hillside, and extensive city walls. The Archaeological Museum holds finds from centuries of excavation. The northeastern beaches — particularly Aliki, where an ancient marble quarry descends into the sea, and Paradise Beach — are the best on the island. The interior has monasteries and mountain villages. Thassos makes a comfortable full-day excursion; it can also be combined with a night's stay if you want to see more.