Bangkok has two transport systems that barely overlap: the river, which serves the old city and the temples, and the elevated and underground rail, which serves the modern one. Three days works best when each day picks one.
Day one: the river
Start at 8am. Heat and crowds both build fast.
The Grand Palace at opening, with Wat Phra Kaew inside. Two hours, and the strictest dress code — shoulders and knees covered, enforced.
Wat Pho, adjacent — the 46-metre reclining Buddha and the traditional massage school, where an hour costs a fraction of a hotel spa.
Wat Arun, a short cross-river ferry from the Wat Pho pier.
Then ride the Chao Phraya express boat — 16–33 baht, public transport, and the best sightseeing in the city.
Evening: Chinatown (Yaowarat) after dark, when the street food stalls set up along the main road.
Day two: the modern city
Everything along the BTS and MRT, which are fast, cheap and air conditioned.
Chatuchak market if it is a weekend — 15,000 stalls, in the morning before the heat.
Jim Thompson House — the silk merchant's teak compound, guided, an hour, and a good air-conditioned midday option.
Lunch in a mall food court — Terminal 21 or EmQuartier — which sounds unpromising and is where a lot of good cheap Thai food is.
Lumphini Park in the late afternoon, with monitor lizards.
Evening: a rooftop bar, with a dress code, or the night markets.
Day three: get out
Bang Krachao — an undeveloped green peninsula in a bend of the river, reached by a ten-minute boat from Klong Toey pier.
Hire a bicycle at the landing and ride raised concrete paths through jungle and gardens. Twenty minutes from the centre and it feels like another country.
Or Ayutthaya — the former capital, sacked in 1767, with temple ruins and the famous Buddha head grown into tree roots. Ninety minutes by train or road, and a full day.
Skip the floating markets on a short trip. Two hours each way for something heavily staged.
Where to stay
Near a BTS or MRT station. This matters more than the neighbourhood, because traffic makes taxis unpredictable and trains do not.
Sukhumvit or Silom, both connecting to the airport rail link.
The old city near the temples is atmospheric and badly served by the train network, which costs time.
Practical
Airport Rail Link from Suvarnabhumi to Phaya Thai, about 30 minutes, connecting to the BTS.
The temple closed scam — someone near the Grand Palace says it is shut for a ceremony and offers a tuk-tuk itinerary ending at a gem shop. It is not shut.
Never point your feet at a Buddha image or a monk.
Book tours locally — a day trip arranged at a shop the previous evening costs roughly half the online price.