I grew up in Athens and here is some stuff you might find useful.
Athens is a big city and it has being withstanding one of the worst financial crises of modern Europe, so watch your stuff, people will steal anything for some dough. Always walk in groups especially in night time.
Try not to get scammed by taxi drivers
Go to skateshops and ask for locals to show you around, you'd be amazed by the abundance of hidden spots
There is a website called thespot.gr which has a decent sized database of spots, i do not know if it is available in english but you can also filter it based on what you want to skate.
If you are staying in Athens for a while, try to go to the island of Andros sometime, where this god sent, skate friendly hotel is situated
http://www.bluenigma.com/Last there is no really need to go to bars, just grab a beer from the shop and head down to the street where Color Skateshop is located, you would be amazed by how many skaters/people are hanging there during night time.
P.S. Go bomb Lykavittos Hill, you'll love it