it was about time to come here :P
The good thing is that in Athens you literally see everything. There are places for every type of person so in one hand it's good you haven't planned anything, just go out and explore! Don't forget to try as many different types of food and drinks as you can, i bet that experience will be unique!
Ahh and don't miss the cafes,clubs,bars and in general the entertainment and nigh life !!
just stay out of Omonoia at night it's not that safe :P
say that again.