Every 10th TFL or public transport charge, a push notification or timeline prompt to suggest walking or cycling instead.
More Moolah, spend calories, Cut the bill down.
Edited to add: The data collated through this could be used to provide a Model to compare one persons spending to the average Mondo user.
Perhaps link that idea in with '‘targets’'if that message pops up suggesting walking and you hit an accept button and enter the station you would have travelled to. Number of times you do that tallys up and works out the saving in £’s you’ve made!
I like the steps idea… could be linked to iPhone accelerometer, or apple health.
Using City Mappers API would provide route suggestions and alternatives across a range of different methods of transport, giving each Mondodian choice/options rather than jus the same old routine.
Given that TfL charge only once a day (normally overnight) there would need to be integration on a new scale before something like this could ever be contemplated.