The Chao Phraya river runs through Bangkok into the Gulf of Thailand, runs through Bangkok might not be accurate as it twists and turns all through the city. There are many great places to visit along the river, this photo was taken from Wat Arun. A trip on the river is a good way to view the contract of new and old structures in Bangkok.
The twists and turns of the Chao Phraya causes many traffic headaches- it prevents Bangkok from a logical street grid pattern and there are a limited number of bridges to cross the river. Visiting the Chao Phraya is easy- the BTS skytrain runs right to the river, you can then jump on water taxis or long tail boats.
I have been lazy about visits to the river, I went last weekend and was reminded how interesting and diverse the river is.