Today I worked out how to use Things as a Kanban system. The trick is to use the "Focus" top-level item on the sidebar properly. A lot of my tasks had built up in the "Next" folder. I moved all those tasks from "Next" into the "Someday" folder. Then I only move into the "Next" folder the stuff I'm planning to work on that day. The task I'm currently working on I move into the "Today" folder.