Bangkok makes sense as a set of connected districts rather than a single centre. The Grand Palace and major temples belong to an Old City and river day; Siam, Sukhumvit and Chatuchak require different rail journeys and different amounts of time.
The Grand Palace and Wat Phra Kaew sit near Wat Pho, while Wat Arun is across the river. Grouping them limits road travel and leaves room for a ferry, museum or flower-market stop.
These are active religious and royal sites with security and dress requirements. Check official opening information and carry clothing that covers shoulders and knees rather than relying on a street approach offering an alternative tour.
Yaowarat and Sampeng form a dense Chinatown trading area; Chatuchak is a separate northern market journey. Siam and nearby Pathum Wan combine malls, art and indoor options that work well in heat or rain.
Use rail and river services for the corridor they serve, then allow for the final walk. A map distance that crosses the Chao Phraya or a major road can take longer than expected.