Drag-and-drop that feels right
The interaction most prototypes get wrong, working properly.
A kanban board with drag-and-drop, labels and due dates.

Trailhead Boards is an interface exercise: the drag-and-drop, the label chips, the checklists and the due dates all behave the way anyone who has used a project tool expects them to.
State lives in the browser, so it is a working prototype rather than a production tool — which is exactly what you want when you are still deciding how a workflow should feel.
Internal tool / prototype. Built to prove an interaction model before committing engineering time to a real backend and multi-user sync.
The interaction most prototypes get wrong, working properly.
Enough structure to run a real exercise on it.
State persists locally so you can demo immediately.