Wat Pho (the Temple of the Reclining Buddha), or Wat Phra Chetuphon, is located behind the Temple of the Emerald Buddha.
The temple is one of the largest temple complexes in the city and is famous for it's giant reclining Buddha that measures 46 metres long and is covered in gold leaf.
To enter, you would have to remove your shoes and all visitors must wear appropriate clothing; meaning no exposed shoulders or skin above the knee.
This is also a great place to get a traditional Thai massage as Wat Pho is often considered the leading school of massage.