The Chao Phraya (Thai: แม่น้ำเจ้าพระยา) is a major river in Thailand, with its low alluvial river plain marking the mainland of the country.

It begins at the confluence of the Ping and Nan river at Nakhon Sawan (also called Pak Nam Pho) in the Nakhon Sawan province. The Nan and its biggest confluent, the Yom River, flow nearly parallel from Phitsanulok till Chumsaeng in the north of Nakhon Sawan province. The biggest confluent of the Ping is the Wang River which enters near Sam Ngao district in Tak province. The Chao Phraya system drains an area of approximately 160,000 km², of which the largest contribution is the Ping with 35,000 km².

The Chao Phraya runs from north to south for 372 km from the central plains to Bangkok and the Gulf of Thailand. However in Chainat the river splits into the main river course and the Tha Chin river, which then flows parallel to the main river and exits to Gulf of Thailand the about 35 km west of Bangkok in Samut Sakhon. In the low alluvial plain which begins below the Chainat dam many small canals (khlong) split off from the main river, used for the irrigation of the rice paddies.

The cities along the river are Nakhon Sawan, Uthai Thani, Chainat, Singburi, Ang Thong, Ayutthaya, Pathum Thani, Nonthaburi, Bangkok and Samut Prakan relatively.

On old maps the river is named as Menam or Mae Nam, the Thai word for river. The name Chao Phraya is also a Thai feudal title, which can be translated as General. In the English-language media in Thailand the name is often translated as 'river of kings'.

Notable Bridges
Rama VI, rail-road bridge of the southern line
Phra Pin-klao, near Grand Palace
Rama VIII, a single tower asymmetrical cable-stayed bridge
Rama IX, semi-symmetric cable-stayed bridge
Mega Bridge, part of Industrial Ring Road

Transportation
In Bangkok, the Chao Phraya is a major transportation artery for a vast network of ferries and water taxis.

Extracted from Wikipedia: Chao Phraya River

Go by BTS to Saphan Taksin. By a 20 baht ticket to Nonthaburi and spend a nice cooling time on the river. Lots of nice sides and Nonthaburi pier as good food stalls.

No need to take the 150 baht tourist boats, take the regular local service. If you get tired of the river, hop off at the royal palace with the national museum nearby and at a walking distance of Khao San Road.

You might like to try China town and indulge in street food.