A few things about visiting Athens:
-You will definitely want to get Euros before you go. While there are ample opportunities to use ATMs and banks all over the city, the conversion rate and extra charges incurred by you could waste more money than you planned. If you are going to be staying there for an extended period of time (such as I did), you definitely want to find the corresponding bank over there. Be prepared to not be able to do that, as they only have 2 or 3 banks that are internationally affiliated.
-Bring an umbrella if you are visiting during the fall. It is the rainy season and boy does it rain! Off and on for a couple of months, so it will definitely be an adventure.
-And on that note...sturdy shoes. I cannot express this enough, bring. sturdy. shoes. Everything is made of marble, as it is and was widely available and most of the city is in tact from when it was first built. Thus, the sidewalks are made of very slick and very uneven marble. While gorgeous, it is not very conducive to walking. Also, sturdy shoes will be needed, because you will at some point be hiking up hill. Unless you decide to stay in your hotel room the entire time, which what would be the point? :P
-And lastly, get ready to live and enjoy a lifestyle that is so different from our own! The Greeks take a siesta period in the middle of the day (sometimes whenever they choose) and the hours are a bit...different from our own. Make yourself very familiar with the local area, so that in case of an emergency, you'll know where to go. There are very few 24 hour places, so bring things that you think you might need. On the other hand, cafes and tavernas are typically open into the wee hours of the morning and can be a lot of fun to stay out!
All in all, Athens and Greece are gorgeous places to visit and well worth the money spent to visit!