The closest beach to Polychrono (Kassandra) is Chanioti, just 3.4 km away. Within 15 km there are 18 beaches; below are the 12 closest to Polychrono, ordered by distance. Photos, map, reviews.
3.4 km from Polychrono

4.0 km from Polychrono

4.5 km from Polychrono

4.8 km from Polychrono

7.2 km from Polychrono

8.1 km from Polychrono

8.5 km from Polychrono

9.0 km from Polychrono

11.0 km from Polychrono

11.3 km from Polychrono

11.7 km from Polychrono

12.0 km from Polychrono

Chanioti, just 3.4 km from Polychrono. The rest are listed below, ordered by distance.
Within 15 km of Polychrono there are 18 beaches, with 25 across Kassandra in total.
Based on user reviews: Kallithea (★4.7), Kryopigi Beach (★4.4), Pefkochori (★4.4).
The beaches are spread out around Polychrono. A car makes exploring much easier — local buses and taxis are limited outside peak season.
Polychrono is on the Kassandra side of Halkidiki, in northern Greece. The nearest airport is Thessaloniki (SKG); from there a car is the most convenient way to reach the Kassandra villages.