We stopped at Ananuri on the way along the Georgian Military Highway in July. The nature here is stunning — turquoise water, green mountains all around, fresh air. Entry is free, you can walk along the shore and take photos at your own pace. It gets crowded during the day, but if you step aside a bit, you can find a quiet spot by the water. Easy to combine with a trip to Kazbek — it's right on the way. I'd recommend stopping for at least half an hour.