I've always wanted to build a bike machine which creates electricity. I used to wonder why they didn't fill gyms with them, then I realized the cost just didn't cut it, it would take about forty years to pay for itself.
One proposed way to *store* energy was to lift a huge weight up (possibly hanging in a mineshaft) when demand was low, and let it down when demand is high. Pumping water up a dam turns out to be a better way to do it, but the principle is the same.
This idea is to take an old Ferris wheel from a fairground, and an old fire escape from a building (or just a ladder), weld them together and use the motor as a generator. People can climb to the top using their legs very efficiently, and can sit comfortably in a carriage which pulls the wheel around. The low gear ratio means it doesn't need to stop (like many big Ferris wheels) and it should generate about the same as a medium sized wind turbine.
People can be paid by the joule, which is great for the obese, as they get paid more for their exercise. Athletes could carry heavy weights with them to get more out of each trip. Built on an enormous scale, it might take over an hour for a full rotation, so people can socialise/read books/browse the web in their carriage as they're making electricity. It could even be built at the edge of cliffs, so to get to the top, hikers can go for a lovely walk round instead of the boring stairs. You could even cycle your way to the top.
The advantages of course are that it's far cheaper on a per-person basis, it's much more customisable to what method of exercise you want to do, and the large motor and heavy flywheel should mean better efficiency than lots of little machines.
You could also have a similar generation function in skyscrapers, for people who take the stairs up, but take the lift down. But maybe that's more effort than it's worth.