To go from the airport to the center of Athens by bus you can take the bus line X95 Syntagma Square- Airport Express. This line is available 24 hours a day and buses from this line departure every 10-15 minutes. The bus station to the President Hotel is "Plateia Ampelokipon" and to Ilisia Hotel is "Ilisia". Below you can find maps, with the route from the station to the hotel. The ticket for the bus costs 3-4 euros.
Plateia Ampelokipon to President Hotel
Ilisia(station) to Ilisia Hotel