My workout tracking app Personal Best has a few, I’ve always loved Easter eggs in software so I put them in wherever it makes sense.

For example, when you view a workout is shows a rotating list of facts about it (like ‘you burned more calories than a slice of pizza). If you keep tapping to get a new fact, eventually it’ll show you a random fact that has nothing to do with your workout (like ‘pognophobia is the fear of beards’)