Vouliagmeni is about half an hour from Athens by car and is a great place for beachside dining. I find that this area has amazing views and is much less crowded than Athens. I haven't been to any beaches there, so please let me know if there are good spots to swim nearby. I usually go for dinner if im staying in Athens and then take a taxi back. My favorite spots are Ithaki and Lavros. Check them out!
Athens has some great nightlife!! There are so many options to choose from, and it really depends on what you're looking for. My personal favourites are rooftop bars that overlook the Parthenon, you can find most of these in Thissio. But many local Athenians would say that these spots are very touristy and not as fun Exarchia and Gazi for example. If you're interested in going to bars that are by the beach, but still pretty close to the city I recommend Gylfada. Psiri it's also a very trendy neighbourhood that is popular amongst the locals! Try them out and let me know what you think!
Year after year, Athens is hitting record highs for temperature. This year, it's getting so hot that we're seeing wildfires again. I highly recommend that if you're visiting the city, that you wear a lot of sunscreen and stay very hydrated. Heatstroke and sun disease are actually very common in Greece, and can be very dangerous. Make sure that you're in the shade, and stopping in restaurants and shops that have air conditioning.
Now hear me out. I usually go to see a performance at the Odeon of Herodes Atticus theatre every time I visit Athens. A few times I have seen operas there. In fact, I have only been to operas at this theatre, having never been to an opera at any other time in my life. I have to say it is an amazing experience. If you are going this year and you have never been to an opera, Toscana is being performed here as part of the Athens & Epidaurus Festival. Do yourself a favour and go see it. You can find details here: https://whyathens.com/events/tosca-greek-national-opera/
And to get you in the mood, here are a couple of photos of the last opera I went to see there: