While doing the sprint planning meeting, very often new feature ideas crops up and if not recorded immediately can be lost. Why not have another white board with post it notes, for the product backlog itself. This gives the team absolute visibility on the shape of things to come, ans their brains also will work on “how to improve the product better”.
In the classtube project, we have three white boards; one for the design to evolve, the second one for the product backlog and the third one for the sprint tracking. It is working well for us.