I was going to say thesoloniki, too. I love that town. also, I'd go to Philips tomb, which is an hour or 90 minutes away from thesoloniki. the food is great.
here's the deal, I'm a lot older than you and not a party guy, Istanbul is a real party city, so I can't help with that. if I were you
3 days in Athens - archeological museum, Cycladic museum, walk around town, acropolis, I'd have a meal at monostaki metro station square, one in the plaka, and one in a fancy greek place.
take a train to thesoloniki - one day in thesoloniki, one day at the ruins of pela and vergina, tomb of Philip.
train to Istanbul
3 days Istanbul, hit a bath, the bazaar, the spice bazaar, blue mosque, grand mosque, haga sofia, walk around town, archeological museum, spend an evening smoking a water pipe, take a ferry to the asia side, eat at a fish place on gulata bridge, have iskender kabab, go to a high end kabab resteraunt.
troy one day
you could play around with the other two days - depending on what you're interested in, I'd probably do some more ruins.