It gets hot in Athens mainly in July and August. Here are some useful tips on what to do in Athens during summer.
Drink a lot of water, carry water at all times with you. Half a litre bottles can be purchased almost everywhere at 50cents per bottle. The heat is not that bad and won’t kill you, but it will try. Try not to walk in the sun and always wear a hat.
In the morning:
As you will most probably visit the Acropolis you should consider doing it early in the morning i.e. 8am. It will be busy but it will be less hot. Now, if you don’t mind the heat then you can go around 3pm when it will be for sure less crowded.
In the afternoon:
As you will be walking around Athens and the archaeological sites eat something light only. It should be enough until your dinner. The single biggest trick: Go back to your hotel and take a siesta nap. You will definitely need this.
If you are still walking around Athens then visit the National Garden. It is a really nice place and for sure much cooler, due to the big trees, and much quieter.
In the evening:
Evening is good too, it is less hot for sure. There are some nice things to do.
Have a walk around the Acropolis, from Thision to Monastiraki (or vice versa). It is a pedestrian territory. Also, very nice and in the same area: Walk up to Filopappou, the green hill next to the Acropolis, you can sit there in the evening when the sun goes down and the lights on the Acropolis get turned on.
Total out of the box alternative: Book a Dinner in the Sky! Yes, a dinner while suspended in the air. Dinner in the Sky a worldwide concept operating in Athens for the last 5 years. You can choose between the Sunset time dinner or the later one and enjoy a 6 course menu prepared in front of you as you are having a once in a life time experience.
There are also open-air cinemas where you may watch some of the latest movies while enjoying a cold beer.
There are many things to do in Athens even during hot days. Of course, you can spend the entire day in one of the island that are very close to Athens. (Aegina, Poros, Hydra) or rent a car and drive to Delfi (2,5 hours) away.