the Grand PalaceConsistently the top recommendation for visitors, and worth allowing a half day rather than squeezing it between other stops.
Local culture beyond the checklistChatuchak Weekend Market and the floating markets outside the city are where Bangkok feels least like a tourist itinerary and most like the actual city.
A well-known second stop: Wat PhoRounds out a first visit nicely alongside the bigger-name sights.
Easy onward travelBangkok's transport connections make it simple to extend the trip into the rest of Thailand if you have the time.