Here is a video that helps explain what a Learning Walk is about. Learning Walks are sometimes referred to as Genius Hour or Passion Projects.
Video: Genius Hour
Here is a good way to think of topics -
Research - if the student wants to learn more about a certain topic
Test - If the student wants to do a science related experiment
Make - If the student wants to produce/make something to show others.
There are links in the next column to help you find ideas.