We'll have breakfast at Harchevnya. It's delicious, nourishing, cheap, and you can sit on a cozy veranda.
I suggest visiting the largest and most famous water park on the Black Sea coast. The well-groomed area with centuries-old pine trees has everything you need for family recreation: cafes, children's bathing area, relaxation area with massage and, of course, extreme slides! You can have a tasty and budget lunch in the large dining room under the awning.
Evening can be brightened up by a walk along the famous Lermontovskaya embankment. I also suggest you take a boat ride and see the panorama of the evening city from the sea.
After the evening moonlight, a snack would be nice. We'll do it at the local pizzeria.
You can spend the night in one of the hotels or guest houses in Gelendzhik.