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.