People to avoid, Athens
Something that happened to me and a friend of mine: we were looking for the tourist office (which was closed), and this guy came up to us in Sintagma Square saying he knew where the tourist office was, but that he had better info for us, all we had to do was walk into a building across the street and he'd give us info about ferries, pensions and stuff. That sounded quite strange to us, so we managed to get away. Of course the guy's reaction (he got quite mad) proved that we were right to do that.
This may sound obvious to those who have traveled around a lot, but for those who are beginners I think it is good that you hear these stories.
The single male traveller will often be approached being offered help or being invited to a bar to make friends. More 'friends' and beautiful women will appear and they´ll eventually end up paying a huge bill at the bar.
I was approached twice in this manner but since I had already been informed about it I didn´t fall into this trap.
This one is for men. Men will enticed you to go to one of the bars or clubs(mostly likely strip joints) around Omonia. In the very least, they will charge you a fortune for drinks and worse; strippers pickpockets you or rob you. I was warned by the owner of my hotel. Surely enough, I was approached by one around the Temple of Olympian Zeus. These hustlers are very keen and read you like a book. They have props, in my case. Business cards of people from where I live. Anything to gain your confidence. I'll give these people an A+ for salesmanship.