Wat Arun, locally known as Wat Chaeng, is a landmark temple on the west (Thonburi) bank of the Chao Phraya river. It's easily one of the most stunning temples in Bangkok, not only because of its riverside location but also because the design is very different from the other temples you can visit in the Thai capital.
Wat Arun (nicknamed the 'Temple of Dawn') is partly made up of colourfully decorated spires and stands majestically over the water.
Wat Arun is almost directly opposite Wat Pho, so it's very easy to get to. From Saphan Takin boat pier you can take a riverboat that stops at Pier 8.
From here, a small shuttle boat takes you from one side of the river to the other.
I'd recommend spending at least an hour visiting the temple. Although it's known as the Temple of the Dawn, it's absolutely stunning at sunset, particularly when lit up at night. Even so, the quietest time to visit is early morning, before the crowds.