Flexbox Froggy is an interactive tool, ostensibly a game, designed to help you learn how to use CSS Flexbox.
The Flexulator tool I’ve linked to before deals with the flex and grow properties of Flexbox. Froggy on the other hand concentrates on positioning using justification, alignment, wrapping and order.
The twenty-four exercises provide a controlled and increasingly-difficult sandbox in which to learn. I might not call them fun, but they were a helpful challenge.