While doing some web searching I ran across a fascinating little company called
EmSense that creates a headset that tracks eye gaze and brain activity. They're marketing it to product marketers, web designers, and video game designers as a way to track what attentional and emotional responses they have as they experience the product/website/game.
This technology could be used to tune these various experiences to be as mentally engaging as possible. However, the device is made to be used by a small number of product testers instead of end users, so it can't capture the full range of responses that different types of people will have.
And end-user version would be interesting. If used with a video game, it could monitor your level of excitement and focus as you play in order to make the game as engaging and fun as possible.