Nancy, needing flat surfaces for planners, notebooks - yes, that can be a problem. I am with you that any 3-ring binder like household notebook or for instance my healthy habit notebook is so helpful BUT isn't level when you set it down temporarily. I'm forever shaking my head over a piece of paper or whatever set on top that slides off.
One thing about cards - that doesn't happen with them. Small space, nothing at an angle.
But right now I don't want to give up the full sheets I'm using for a few things. So I may try setting up a vertical place, setting the notebooks as if they are books between bookends. Thinking.