Bangkok is already well known for sky bars like Sirocco and Red Sky. View Arun rooftop bar is a bit more low key since it is only on the 5th floor. But it has a great terrace facing directly towards the iconic corn/pineapple-shaped prang of Wat Arun. Arguably the best view of Wat Arun that you can get. To me, Bangkok is all about river life. I love to stay in hotels like Chatrium and ibis and watch the traffic and life on the river of kings, Chao Phraya. View Arun terrace books up pretty fast.
Therefore, it can be a good idea to ask for good spots a week in advance. Check the time for sunset and come 30 minutes before. At my time the sunset was about 5.45 pm. The terrace was fully booked from 5.30 pm. The thing is that the temple tower is lit up after sunset also.
Another aspect is that you also have a nice view of the towers of Wat Pho and Grand Palace in the distance.
After I visited Arun View, I walked to the flower market, Phak Klang Talat. This was a real eye-opener. I have only been there in the morning and daytime before. But since it is a 24 h market, the activity in the evening was really interesting.
Flowers are sold for religious rituals all around Bangkok so the market is constantly bustling with activity.
After this small evening adventure, I returned to Siam Square for a good rest at Novotel.