Get early and head to Grand Palace in the morning at the opening around 8:30am to avoid the crowds. You can walk around the palace for 1-2 hours without any people around, this is priceless!
Once you finished visiting the Grand Palace, walk to Wat Pho, it's just a 10 minutes walk. Visit the reclining buddha of course, but don't forget to walk around the temple, there are many other wonders to see.
After wat Pho, head to Tah Tian pier and take a short boat ride to Wat Arun (cost around 4-10 THB only). Enjoy this beautiful temple and climb on top for a great view of downtown and the Chao Phraya river.
After Wat Arun, take the tourist boat (30-60 THB) to icon Siam, the newest and biggest Mall of Bangkok. There are tons of restaurants, bars, and shopping opportunities, they also have a floating market and some great fountain show at 4pm, 6:30pm, 8pm. Don't forget to check the view of Bangkok skyline from the terrace at the 6th floor.
From Icon Siam, take a taxi (use the mobile app Grab or Bolt) for Chinatown. if you have time, check the golden buddha at wat Traimit and the Chinatown gate, then walk slowly to Yaowarat road. If it's already the evening, head directly to Yaowarat road by taxi. Chinatown is a street food paradise, so don't hesitate to try many dishes. I recommend all the stalls with the sign "Michelin guide". You can also check Widdershins, a secret bar offering the best view of Chinatown.
Lastly if you still have some energy left, take a short taxi ride to Khao San road, where you can have a drink in one of the hundreds bars in this party street.
