Sithonia is the middle leg of Halkidiki, the three-pronged peninsula south-east of Thessaloniki, and the one with the reputation for being calm. Kassandra to the west is the busy, party-minded one, and the Mount Athos peninsula to the east is a monastic republic. In between, Sithonia is hills of pine and olive, a mountain called Itamos in the middle, and a coastline of sandy bays that has been, for a few decades now, a summer favorite of families from northern Greece and the Balkans. Its history shows in odd places. The ancient city of Torone, near the southern tip, was fought over by Athens and Sparta in the Peloponnesian War, but the towns you'll sleep in are young: Neos Marmaras and Sarti were founded by refugees from the islands of the Sea of Marmara in the 1920s, and Porto Carras, the giant resort whose construction began in 1973 on land planted with vines since the late 1960s, helped turn the peninsula into a holiday destination.
This is a beach holiday with five good days in it, and it's better to say so plainly. If you're staying a week or two, you'll spend most days on whichever beach you liked best, and you'll know which one that is by day three. This plan gives you the other five: one corner of the peninsula per day, each with a beach to swim at, something to look at and a table worth sitting at. The days work from any of the five towns people actually stay in (Nikiti, Neos Marmaras, Vourvourou, Sarti and Toroni), and each is within roughly an hour to an hour and a quarter's drive of any of them. You will want a car. The buses that serve the peninsula are fine for getting between resorts, not for doing what's below.
Day one is the middle of the peninsula: a shallow beach, a stone village in the hills and a winery. Day two is the south-west, Toroni and Porto Koufo. Day three is the south-east, from Sarti down to Kalamitsi. Day four is a boat along the Mount Athos coast, and day five is Vourvourou, the Diaporos islands and old Nikiti. Only two things are tied to the calendar: some Athos boats don't sail every day, so ask about the day first and fit the other days around it, and the ancient site of Olynthos in the "more time" list is closed on Tuesdays. Anywhere else, start whenever you land. If you're flying into Thessaloniki, a night there is worth it too, and our 5 days in Thessaloniki covers it. More on the country is on the Greece page.