I first learned about short self-assignments from Natalie Goldberg's well-known Writing Down the Bones. Goldberg introduced the concept of "freewriting" to us with that book, and many writers discovered new energy to sail past writing stall-outs by giving themselves freedom to write small, short, and random.
Working on a book project often brings me a sense of being so overwhelmed, I can't think of anything to write about. I make brainstorming lists of topics, and this helps. But sometimes I have to write outside my story, just to get the momentum going again.
Short self-assignments help tremendously.
In her book, Thunder and Lightning, Goldberg told the story of a time when she and a friend were stuck, unhappy, and unable to think of how to move forward creatively. They tried talking. They tried taking a hike. But nothing worked until they both sat down and did a timed writing session. As I remember the anecdote, they picked a topic outside their current writing projects, something that had less importance or weight, and this freed up the stuckness.