Hi there "Onur".
End of August will be busy indeed both in Kassandra and in Sithonia. Now, regarding Sithonia that I can advise you for sure, there is a really long coastline that will definitely not be busy anywhere. So, even in August there are places you can relax with not big crowds. If you don't mind a short drive, except from Neos Marmaras, I would also advise you to check on Parthenonas, which is a small traditional village/settlement, just 5km away from Neos Marmaras , overlooking the sea and offering less busy days, and definitely cooler nights. There are a few nice B&B's to be found there, as well as a couple of restaurants, two cafes and generally a relaxed atmosphere. Around Neos Marmars, you will also find lots of different solutions regarding accommodation, I would mostly advise you to check the surrounding of Neos Marmaras, instead of the town center. Having your base in the area, you could easily approach other nice resorts as well (including Nikiti, Porto Koufo, Toroni, Vourvourou), you could have access both from Neos Marmaras and Ormos Panagias to beautiful boat trips, and also be close to other activities such as wine tasting, trekking, mountain cycling, scuba diving etc.
In case you need any further help, please don't hesitate.
Regards,
Giannis