Wat Po is one of the oldest temples in Bangkok. It is most famous for the Reclining Buddha - though Wat Ratprakongtham claims to have an even bigger one! Many tourists only go here to view the Reclining Buddha; don't do this, take the time to wander around the whole complex. As you can see from these pictures there are many other areas worth looking at.
A famous school of traditional Thai massage is based at Wat Po - the brave and hardy can make use of their services.
Chinatown and the Grand Palace are nearby if you want to combine other sites on a day tour. Tha Tien is the best River Taxi Boat landing stage for Wat Po.