Bus: Take the X95 bus to Syntagma
Taxi: taxis are stationed right outside the arrival gates at the airport. There is a fixed flat rate from the airport to the city center (and vice versa), costing approximately 43 euros during the day and 59 euros in the evening (00:00 – 05:00).
Train: Take the metro (blue line) to Syntagma

Bus: the 040 bus begins and ends its route at Fillelinon str, at SyntagmaFrom Larissis central train station
Taxi: there are numerous taxi stations around the port.
Train: take the metro line 1 to Monastiraki square, then change there for line 3, towards Syntagma.

Bus: Outside the airport you can take the Bus Line X95
Taxi: there is a taxi station outside the train station.
Train: You can take tha Proastiakos Line to Liosia and Metro to Syntagma