Two days covers the riverside temples if you start at opening. Three days is the comfortable minimum for Yaowarat without treating the BTS as the holiday.
2 days: temples and the river
Day one: Wat Pho and Wat Arun, ferry dusk, dinner near Tha Tien. Day two: Grand Palace at 8:30am, then Chatuchak if it is a weekend or Yaowarat if it is not. Skip Ayutthaya.
3 days: city plus neighbourhood
Keep the two-day core, then give day three to a slower market morning, Thip Samai, or a river hotel afternoon with no major ticket. This is the version that still feels like Bangkok rather than a queue.
5 days: Bangkok and Ayutthaya
Three city days, one Ayutthaya weekday (early train, bicycle, ruins), and one buffer for heat or a second food night. Stay in Silom, riverside, or Sukhumvit so you are not commuting across the city each morning.
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