This is one of our favorites. It’s not the most popular one in Bangkok, but especially for first-time visitors this temple offers all you need: beautiful architecture, local religious activities, a great gallery with dozens of Buddha statues and an overall relaxing atmosphere right in the middle of Bangkok. Wat Suthat is situated centrally, right next to the Giant Swing (you might want to google that).
The area around the temple is also great for another first encounter with Bangkok: right outside the temple walk the small alleys where Buddha statues are being manufactured and renovated. You will love it.