The Chao Phraya River, the River of Kings (in Thai) flows through Bangkok passing many interesting sights along the way. Taking a boat trip is a good sightseeing experience with the city looking different when seen from the river.
The best option for a day-time trip is to take the Chao Phraya Express Tourist boat from Sathorn pier to its final stop at the Phra Athit Pier (or vice versa) at a cost of 60 baht for a single ticket.
The tourist boat includes an English-speaking guide who gives a commentary pointing out places of interest along the way and there is a small outside deck at the front of the boat. The tourist boat is usually less crowded than the normal Chao Phraya Express Boat (orange flag) service.
On the journey, you will pass temples, churches, Chinese pagodas & shrines, colonial-style villas, some of Bangkok’s top hotels, skyscrapers, forts, former royal palaces, and riverside parks.
Other day-time options include taking an organized tour. These operate from the major piers or you can hire your own private long tail boat. There are also cruises up the river to Ayutthaya.
In the evening, there are several popular romantic sunset dinner cruises. They sail up and down the river making a slow circuit of the riverside sights.
For more information on river cruises click here: https://www.touristbangkok.com/tours/