Forty-eight hours in Bangkok works well, provided you accept the geography: the old temples are on the river and the modern city is along the train lines, and moving between them at the wrong hour costs an hour in traffic.
Where to stay
Near a BTS or MRT station. This matters more than the neighbourhood, because Bangkok traffic makes taxis unpredictable and the trains are not.
Sukhumvit or Silom — both connect to the airport rail link and to the river.
The old city near the temples is atmospheric and poorly connected to the train network, which costs you time on a short stay.
Day one: the river
Start at 8am. The heat and the crowds both build fast.
Grand Palace at opening, with Wat Phra Kaew inside it. Roughly two hours.
Wat Pho, adjacent, with the reclining Buddha and the traditional massage school — a proper Thai massage here is worth the hour.
Wat Arun, a short cross-river ferry from the Wat Pho pier.
Then take the Chao Phraya express boat north or south — it is public transport at 16–33 baht and the best sightseeing in the city.
Dress code at all three: shoulders and knees covered, shoes off where indicated. The Grand Palace rents coverings and enforces this strictly.
Evening: Chinatown (Yaowarat) after dark, when the street food stalls set up along the main road.
Day two: the modern city
Chatuchak market if it is a weekend — 15,000 stalls, and go in the morning before the heat.
Otherwise: Jim Thompson House, the silk merchant's teak compound, which is guided and takes an hour.
Lunch at a food court in one of the malls — Terminal 21 or EmQuartier — which sounds unpromising and is where a lot of good, cheap Thai food is.
Afternoon: Bang Krachao, the undeveloped island reached by a ten-minute boat from Klong Toey pier. Rent a bicycle and ride the raised concrete paths through the greenery. It is twenty minutes from the centre and feels like another country.
Evening: a rooftop bar — there are many, dress codes apply, and the view is the point — or the night markets.
What to skip on 48 hours
The floating markets. Two hours each way and most of a day, for something heavily staged.
Ayutthaya. Excellent and a full day.
The outlying temples. The main cluster covers it.
Transport
Airport Rail Link from Suvarnabhumi to Phaya Thai, about 30 minutes, small fare, connecting to the BTS. Taxis take 40 to 90 minutes and cost several times more.
BTS and MRT for everything they reach. River boats for the old city.
Grab for the gaps. Metered taxis are fine if the driver uses the meter; if they quote a fixed price, take another.
The scam to know
Outside the Grand Palace and Wat Pho, someone will say the site is closed for a ceremony and offer an alternative tuk-tuk itinerary.
It is not closed. Walk to the official entrance.