You have one day. Assume heat by 10am, a brief downpour somewhere between 14:00 and 16:00, and a city that empties its kitchens onto the street after dark.
Start at the Grand Palace before the tour buses do — gates open at eight, the light on the mirrored chedis is still soft, and you will clear the compound in about ninety minutes if you don't double back. Next door, Wat Pho keeps a reclining Buddha that fills its hall like a train in a tunnel; the massage school at the rear is cheap and excellent if your feet are already sore.
From Tha Tien pier, a flat-bottomed ferry crosses the Chao Phraya to Wat Arun in under four minutes. Climb the central prang if the rails aren't busy — the view back over the palace is the one on the postcards. By mid-afternoon, retreat to air conditioning (a mall on the BTS line, or a café in Talat Noi) until the sun lowers.
After 18:00, take the MRT to Wat Mangkon and walk south onto Yaowarat. The chefs set up at 18:30, the smoke thickens by 20:00. Eat standing. Finish on a rooftop if the night is clear.
