Calorie/exercise tool for gyms ... ? Ideas pls.

I'm considering creating a jQuery/Css tool to estimate how much gym activity you'll need to do in order to burn off a selected meal from selected local takeaways.

Rather than be 'preachy' I want to add a gameification element to it but I'm unsure how to proceed.

Also I want the aesthetic and UI/UX to be as engaging as possible. Does anyone have any examples of great JS tools that I can get ideas from?

Eventually the idea would be to roll the tool out to gyms. Who may or may not collect email addresses.

End goals would be to get as many shares and eyeballs as possible while gaining traction in the local online/offline press.

Any ideas?