If you are planning to take the bus, trolley, and metro several times in one day during your stay, buy a 24-hour ticket for all the urban network (€4) or a single ticket (€1.40) valid for all travel completed within 90 minutes. A pass also saves you the hassle of validating tickets numerous times.
Or opt for a weekly pass (€14), or a monthly pass, available at the beginning of the month from terminal kiosks and metro stations (€20 for unlimited bus, trolley, and tram travel; €45 with metro included, not including the transportation to and from the El. Venizelos airport). You need a passport-size photograph of yourself for the pass.