Nightlife in Greece lasts till the early morning hours so many shops and the metro adjust to the program of the locals. There are almost endless things to do at nights depending on your mood. If you prefer relaxed discussion with friends over food and drink you can go to a taverna or mezedopolio (for wine or ouzo drinking with mezedes). If you want to add dancing you might want to check out a rebetadiko, where you can listen to old Greek songs.
If you just want to have a drink, go to one of the bars that can be found in abundance in almost every area. Water is offered for free and most of the time, snacks like potato chips and beer nuts are offered as well. Ordering as a local is something you learn through this course and you will definitely need it!! Gia mas! Cheers!