01 The Acropolis & Parthenon
Entry from €30, and the decision that matters is the hour, not the ticket: there is no shade on the rock, and between June and September anything after nine in the morning is genuinely hard. The combined ticket covering the Agora, Hadrian's Library and the Roman Forum costs little more than the Acropolis alone and stays valid for several days.
02 Acropolis Museum
The counterpart to the rock and the better visit in bad weather, with the Parthenon frieze laid out at eye level on the top floor. Go after the site rather than before, so you have seen the building the sculpture came off. Tickets are usually available on the day.
03 Temple of Poseidon at Sounion
From €30 as a half-day trip, and the only Athens excursion whose timing is fixed by the sun: the point is the temple at sunset over the Aegean. Coaches leave mid-afternoon and get back late. Not worth doing in the morning, when it is a ruin on a headland like any other.