Greece in July is invariably hot, averaging 81 Fahrenheit in Athens (but reaching an average high of 91.5 Fahrenheit in the afternoon). Many of the islands, plus the mountainous areas, can be a little cooler. This is dry season, too, with little chance of rain.
In July the Temple of Apollo - Portara, Naxos, Greece is a high of 81 Fahrenheit, Low 73 Fahrenheit
The tap water is not potable throughout the country. Although it is perfectly safe to drink water from the tap in Athens and Thessaloniki it is better to ask in all the other places or even better buy bottled water which is fairly cheap.
Sewage pipes are very narrow and clog easily everywhere in Greece; nothing but human waste should be flushed.
Greece is a party to the Schengen Agreement; therefore, U.S. citizens may enter Greece without a visa for stays of up to 90 days for tourism or business purposes. ETIAS Authorization may come into affect in 2024: There may be a fee to travel and an ETIAS (European Travel Information and Authorization System) authorization may be required. The Fee should be 7-euros for the application and it should be approved within 96 hours or less, some individuals may take up to four weeks. Below are two links you should navigate to get updated information. (updates coming Oct. 2023) https://travel.state.gov/content/travel/en/international-travel/International-Travel-Country-Information-Pages/Greece.html#:~:text=Greece%20is%20a%20party%20to,for%20tourism%20or%20business%20purposes. https://www.afar.com/magazine/us-citizens-will-need-a-visa-to-travel-to-europe
Here is a video of the town walk to La Portara. Minute 22 is where cars can no longer drive, and minute 25 is where the steps start. https://youtu.be/UukcU577qUk?si=jbYP6C2qN7uJ5nl6