When I first started to describe Game Thinking, I talked about gameful design or game-inspired design. Part of me was always split about what I really meant. In my mind, these ideas were based on user interface more than anything. So creating menu systems that mirrored ideas seen in games, or creating slightly more fun look and feel.
It wasn't until I was messing around with the Snapchat interface that I realised what I was really thinking about - playful design. Design ideas that add to the pleasure of using something whilst not necessarily altering the functionality. The example that Snapchat gave me was something I discovered just by playing with the interface. If you go to your chat stream and slide up, the image at the bottom becomes an animation. At the moment it is of the Snapchat ghosts playing football.
There is no need to do this, it is just a bit of fun. It is a playful reward for exploring and messing with the interface and one that I totally appreciate!
Reward your users for curiosity and exploration or just playing, it may not directly relate to what you want them doing, but it does endear them to the product!