The easiest way to go from the airport to the center of Athens is to take the subway(metro) with direction to Aigaleo. The first departure is at 6:30 and the last at 23:30. The ticket costs 8 euros. The metro station to the President Hotel is "Ampelokipi" and to Ilisia Hotel is "Megaro Mousikis". Below you can find maps, with the route from the station to the hotel.
Megaro mousikis to Ilisia Hotel
Ampelokipi to President Hotel