From Tyler Cowen’s Mar­kets in self-constraint:

A Dan­ish gym chain is now offer­ing mem­ber­ship free of charge, with the only caveat that you have to show up, in order for the mem­ber­ship to be free. If you fail to show up once per week you will be billed the nor­mal monthly mem­ber­ship fee for that month.

As one com­menter points out, the UK health insur­ance firm PruHealth does one better:

By lead­ing a healthy lifestyle (doing such things as buy­ing more fruit and veg­eta­bles and less processed food at the super­mar­ket, or pur­chas­ing sport­ing goods off eBay) not only does your insur­ance pre­mium get dras­ti­cally reduced (in some cir­cum­stances by 100%), but your gym mem­ber­ship fees are also reduced the more times you visit… until it too is free.