This is the world of concepting workshops, whiteboards, sketchbooks and simple prototypes. Below is an activity categorized sampling of the sort of projects I have had the pleasure of working on throughout the years.
Some of the most interesting ideas have come on Post-its, or through workshops with engineering and product management teams. Creativity and ideas are everywhere. In the concept phase, fat sharpies and whiteboard markers are key.
Trying to explain how it works is never as valuable as actually making it try and work. Even if all you are using is paper, valuable insights are gleaned from testing interactions, screen transitions. It is also a very quick and cheap way to test with users.
Higher fidelity than a sharpie concept, but still quick enough that multiple versions and ideas can be explored with very little effort.