Temple of Olympian Zeus, Athens

While it's the largest temple in Athens, the remains are scattered and with no clear historical tidbits. While we were there, half of the columns were under maintenance and cannot be seen. The place bears huge significance and there is great potential for it to be one of the top tourist destinations but requires some work which is still ongoing.

Buy tickets in advance, and if you're short on time, concentrate on the other sites. This one was probably great at some point, but now it's just a few giant colums and restoration works. I'd rather have it restored to it's former glory than walk in an empty area with nothing to see or do :/ But if you have time, you can visit it while going to the biggest park in Athens.