Don and I prefer to take the bus directly from the terminal of arrival to
Syntagma. (Let the driver know that you want to disembark at Syntagma.)
There's no hassel on the fare, which we have found problematic with taxis,
the price is reasonable and the trip doesn't take much longer because most
passengers go to Syntagma or beyond. Have a great trip.
Be sure to obtain some Greek drachmes in the airport to pay for your trip
into Athens (at one of the bank exchange windows).