Walk from Syntagma and the Greek parliament through Ermou street, take photos of the small kapnikarea search, buy some souvenirs on the fleamarket and reach monastiraki square.Follow the adrianoy street behind the metro station and enter the ancient agora and from there up to Acropolis.Afterwards behind acropolis to the Hadrian arc and the temple of Olympius Zeus.Go through the national garden and you are back to Syntagma! :)
Go for a nap and in the night start from Monastiraki to Psyri area and Thisio :) Lots of bars and Ouzeris to decide where you want to have a drink :D