When you can tear yourselves away from the beaches, there are archaeological, historical, religious and nature sites galore. On Kassandra make sure you visit Afitos where you'll find ruins, springs, a superb museum and churches. Visit the Sanctuary of Amon Zeus in Kallithea and make the trek to Nea Fokea for its Wild Life Reserve.
On Sithonia, visit Toroni village for the Paleo-Christian Basilica of Agios Athanasios and the remains of Toroni Castle, head to Neos Marmaras where you can start a lovely walk into the pine forests or visit the nearby Porto Carras for its winery and stables.
The Mt Athos Peninsula is very beautiful - but as it's under monastic control, women are not allowed to visit. Sadly You're missing out on hermitages, numerous cloisters and monasteries - with superb artwork, basilicas and collections of icons, gold and silver. A very small number of male visitors are allowed to visit so book ahead at the relevant monastery.
Those who feel like a break from the beaches on their holiday in Halkidiki, should head to the vineyards of Central Halkidiki. Join an organised tour of any number of local wineries and try the local Sauvignon Blanc and Merlot. Ask at a local hotel's reception about how to join a tour. Or travel to the island of Kelyfos by boat from Neos Marmaras and take a tour of its ancient Roman ruins.