(Previous Post City Hopping at Terminal 21: Rome and Tokyo)
Out of all the floors in Terminal 21, 5F has got to be the floor where I have spent most of my time in. It houses "Pier 21", the food court. As I have mentioned on a previous post, I think this food court best or one of the best in Bangkok.
Try to get a table near the big glass wall/window. That area has a nice view of the buildings along Asok.