The Grand Palace and Wat Arun are the obvious must-sees, and dressing modestly (covered shoulders and knees) is required to enter. Chatuchak Weekend Market is enormous and genuinely worth a few hours if you're in town on a weekend. A Thai cooking class is a fun, hands-on way to spend an afternoon, and the food itself β from street stalls to fine dining β is one of the main reasons people fall in love with the city.
For a break from the heat, the Mahanakhon SkyWalk offers sunset views over the whole city, and a day trip to Ayutthaya's ancient ruins is an easy add-on if you have a spare day.