Interrupting the user flow with interstitials that are not well explained and seem disjointed or tacked on is a surefire way to confuse users and stop them from doing what they're on your app to do in the first place.
Instead, think about fun from the very first design meeting. Make sure each interaction on your app is pleasurable and coherent. This requires thinking ahead, but it's more than worth it.
The simplest expression of gamification, and the one people usually think of first, is also the least useful. Of course I'm referring to badges, points, and leaderboards.
If the rewards are not meaningful, they can't motivate anyone. Imagine someone has asked you to do something you don't want to do, for instance, alphabetizing a bookshelf; a sticker as a reward would not increase your motivation much.
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