[Sweet_Kicks]

All depends on your budget and interests mate, and also when/if you'll come back, if you have 2 months after Athens and want to cram as much as you can in, I'd just go for some of the major cities - Barcelona, Madrid, Berlin (Herzo maybe), Amsterdam, Ibiza (if you want a party week), Paris, London, Manchester, Glasgow, Dublin, Milan, Rome, Copenhagen, Stockholm, Helsinki to name but a few

Festivals are normally sold out ridiculously quick/early, but you might get lucky.