We stopped at Ananuri on the way along the Georgian Military Highway in July. The fortress sits right by the water with mountains behind it — a very picturesque spot. Inside there's an old church, and you can climb the towers for views over the reservoir. Entry is free. It gets crowded, especially during the day, but the grounds are large enough. Easy to combine with a trip to Kazbek — it's right on the way. I'd recommend making a stop.