Why go
A summer visit to open archaeological sites such as the Acropolis needs water, sun protection and a realistic pace. On roads, give traffic your full attention whether driving or walking. At military facilities and museums, follow posted photography restrictions instead of assuming the rules are the same everywhere.
The full story
Safety advice is most useful when it points to a choice you can make. Greek summers can make an exposed archaeological visit tiring, especially on a hill or among stone ruins. Carry water, use shade where available and move the most demanding outdoor stop away from the hottest part of your day when possible. Crowded transport and busy visitor streets deserve the same attention to bags and phones that you would give them in any large city. Road travel is another place to stay alert. Allow time rather than rushing between regions, and pay close attention at crossings and on unfamiliar roads. If you photograph a military or other restricted site, a mistake can have consequences; obey the sign on site. Museum rules can also differ between rooms and collections. Conditions vary by place and season. Use current official advice for changing risks, and make decisions based on the activity, crowd and weather you actually face.