What's the must-see thing in Athens?

Gates open at 08:00 from April through October. For the first 45 minutes you might have the Propylaea gateway nearly to yourself, and at that hour the Pentelic marble catches a warm ivory tone that photographs tend to miss. The hilltop gets hostile fast. By 11am the exposed limestone radiates enough heat to feel through your shoes, the narrow paths between the Erechtheion and the south wall fill shoulder-to-shoulder, and the 5th-century BC Caryatid porch becomes a bottleneck where guides park groups for 10-minute photo rotations. The Caryatids up there are casts. The originals sit in the Acropolis Museum, 6 minutes downhill. Peak-season combined tickets at €30 cover the hilltop plus 6 other sites over 5 days, dropping to €15 in winter.

Walk downhill from the south slope exit to the Acropolis Museum on Dionysiou Areopagitou, a 6-minute descent on smooth stone pavement that gets slippery when wet. The museum opened in 2009 and holds around 4,000 objects from the rock, including the original Caryatids in their climate-controlled glass case on the first floor. The top-floor Parthenon Gallery is the reason to come. It reconstructs the temple frieze at 1:1 scale, with 160 metres of carved marble displayed at eye level. Gaps in the sequence mark where the British Museum still holds Lord Elgin's removals. Glass walls frame the Parthenon directly above, so you read the sculptural programme while looking at the building it came from. Ticket runs €15. Air conditioning and the €4 ground-floor coffee make it the right midday stop when the hilltop is hitting 38°C.

Skip the Temple of Olympian Zeus. It has 15 standing columns, no shade, and no interpretive context beyond a single information board. Spend that hour at the Ancient Agora instead. Enter from Adrianou Street in Monastiraki. The Stoa of Attalos, rebuilt in the 1950s by the American School of Classical Studies, is the only fully reconstructed ancient Greek civic building in Athens. Its ground-floor colonnade stays cool even at 35°C outside. The small museum inside holds 6th-century BC bronze ballot tokens that Athenians used for jury selection, all covered by the combined ticket. The site is shaded by olive and pomegranate trees, with turtles in the drainage channels near the Church of the Holy Apostles, an 11th-century building at the south end. You can sit on a stone bench and hear cicadas and the occasional motorbike echo from Thissio.

Beyond the Acropolis cluster, Athens opens up fast. Plaka, the old town curling around the north slope, has narrow streets lined with bougainvillea and tourist-priced moussaka for €12 to €15 a plate. Worth noting, the better food sits 5 minutes north in Psyrri. The tavernas around Agia Irini Square serve grilled octopus with capers, and the crowd turns mostly Greek after 21:00. Exarchia, 15 minutes northeast of Monastiraki on foot, has the city's densest street art, ouzo at around €3 a glass, and a political edge that shows in the spray-paint and the occasional whiff of tear gas lingering from protest nights. It tends to feel safe during the day. For coastline, the tram from Syntagma runs south along the Saronic Gulf to Glyfada in about 40 minutes, €1.20 on the same transit card as the Metro. Lake Vouliagmeni, a collapsed sea cave with mineral water at a steady 22 to 24°C year-round, charges €15 entry and sits a short bus ride past Glyfada.

A workable sequence for one day. Start at the Acropolis when gates open, spend 90 minutes on the hilltop, then walk down to the Museum by mid-morning. Stay in the air conditioning until the heat peaks around 12:30. For lunch, walk 200 metres west of Monastiraki toward Thissio, where the tavernas serve locals rather than the tourist-trap stretch near the Metro entrance. Hit the Ancient Agora at 14:00 when coach groups have cleared out. The entire loop stays within 800 metres of Monastiraki station on Line 1 (green) and Line 3 (blue, direct from the airport for €9). If you have a second day, the National Archaeological Museum in Exarchia needs its own half-day. It sits 2.5 kilometres north of the Acropolis. The Antikythera mechanism is there. Divers pulled it from a shipwreck off the island of Antikythera in 1901, and its 30-plus interlocking bronze gears tracked lunar cycles and eclipses over 2,000 years ago.