I think the problem with the metro is that there is only a train every 30 minutes, if you miss one, your journey takes 30 minutes longer. On a Saturday morning, the fastest way is probably a taxi. You have to weigh $$ savings by taking the metro vs potential time gain when on a tight schedule. To go back to the airport, be aware that not all taxis will take you where you want to go. Athens was the city I've seen more taxis than anywhere else refusing taking pax if they are not going where the taxi wants to go (of course this happens only when you hail down a taxi on the street, when taking from a dedicated waiting spot, you won't have this problem). 2.5-3 hours for the Acropolis should be sufficient, but will not allow you to see many things around, but I'd prefer this over sitting in an airport lounge for a couple of hours.