This is based on your first full day in Bangkok, where you will start this trip. Visit The Grand Palace first thing when they open at 08:30 am to catch it as quiet as possible as this is the main tourist attraction in Bangkok and it gets VERY busy VERY quick. I know this is not a hidden gem, but if you have not been before it is still worth visiting the main spots of Bangkok to experience the city, the culture and get a taste of the incredibly beautiful temple architecture.
After exploring The Grand Palace walk 10 min to Wat Pho which has a huge reclining Buddha and is overall very beautifully decorated with plenty of flowers.
Now you've probably gotten quite a big dose of people so let's head to the quiet gem of Wat Ratchabophit that is so close to the main attractions and still completely peaceful. We went there twice midday just to get some peace and quiet and it is lovely. The details are absolutely mesmerising and it is so serene.
Next you might want to find somewhere for lunch, just stroll around town for a while and then 1.5 hours before sunset head to Wat Arun. Visit the temple first and then take the ferry across the river again latest 30 min before sunset to watch the sunset over Wat Arun from the other side.
If you still have energy left take a Grab to China Town. It's busy, it's hectic and it's lively, but it's a very unique area of Bangkok. It's a great place to try food from the markets that come to life every night!