If you have not eaten yet and you want to try one of the bistro my recommendation is always: Do it as the Thais do it. If you see a lot of Thais having lunch or dinner in one place, then this is most likely a good sign for good food. By the way, there is a saying about Thais, that if they are not actually eating, there are thinking about what they are going to eat next.
The popular restaurant near Wat Phra Kaew is Khun Deng Guay Jub Yuan and Roti-Mataba..