Wat Arun is the most photographed temple in Bangkok, its 200 foot spire rising out of the city to the touch the skies. If you are in Bangkok, you cannot give it a miss. Visit it in the evening, when its glittering avatar lights up the Chao Phraya River.
If you have the heart for it, climb to the top of the spire for an unadulterated view of the river and the nighttime Bangkok skyline. It is steep and can look scary, but the reward is worth it.