The biggest and most famous Temples in Bangkok are Wat Phra, Wat Pho (lying Buddha) and Wat Arun.

If you have the time I suggest you to book a one day tour to Ayutthaya to see some of the historical parts of Thailand. Ayutthaya, the old capital of Thailand has around 20 different Monuments.

Make sure to book a tour with this 3 stops:

Wat Mahathat (pic left) the oldest and most gorgeous Monument in Ayutthaya

Wat Yai Chai Mongkon: One big chedi and rows of Buddha statues. Really peaceful place

Wat Chai Watthanaram: this area is mystical, you literally think being in a different time-zone