The closest beach to Nikiti (Sithonia) is Nikiti Beach, just 1.6 km away. Within 15 km there are 28 beaches; below are the 12 closest to Nikiti, ordered by distance. Photos, map, reviews.
1.6 km from Nikiti

3.3 km from Nikiti

3.8 km from Nikiti

5.5 km from Nikiti

5.6 km from Nikiti

5.7 km from Nikiti

5.8 km from Nikiti

6.1 km from Nikiti

6.3 km from Nikiti

6.3 km from Nikiti

6.8 km from Nikiti

Nikiti Beach, just 1.6 km from Nikiti. The rest are listed below, ordered by distance.
Within 15 km of Nikiti there are 28 beaches, with 69 across Sithonia in total.
Based on user reviews: Nikiti Beach (★3.8), Trani Ammouda (★3.8), Ormos Panagias Beach (★3.8).
The beaches are spread out around Nikiti. A car makes exploring much easier — local buses and taxis are limited outside peak season.
Nikiti is on the Sithonia side of Halkidiki, in northern Greece. The nearest airport is Thessaloniki (SKG); from there a car is the most convenient way to reach the Sithonia villages.