One of the main attractions in Bangkok is the famous Wat Pho temple. If you want to get a great current impression what it’s like before you go there, you should read this trip report by docgelo. You’ll also find plenty of beautiful photographs there.
Small note: they wrote the reclining Buddha at Wat Pho is the biggest in Thailand, but that’s not true. It’s the biggest in Bangkok, but there are bigger ones in Thailand.