Zebra crossings will not guarantee that drivers will slow down or stop to let you cross the moment you put your foot on the asphalt. Always double-check before you cross and look both ways, even if it’s a one-way street.
In general, you are expected to be respectful within churches, and that begins with how you dress. A T-shirt and pants will do in 90% of the cases.
The Greek sun is relentless and will cause you pretty extensive and heavy sunburns if you don’t respect it. Don’t sunbathe during midday hours, and definitely don’t do so without ample amounts of strong sunscreen.
Not only in Athens but in most of the other cities and villages, the sewage system is old and worn. Flushing paper or, even worse, sanitary products is a surefire way to clog up the system, and nobody wants that.
It may be tempting to take a stone or pebble from the area as a keepsake or a souvenir. Don’t do that. It not only may get you in trouble with the authorities, but it is ultimately hurtful to the site and disrespectful to the country you are visiting.