Lots of other amazing folk are working on this, too!
In March of this year, a design student received a grant to build a prototype see-saw generator in Africa: http://news.bbc.co.uk/2/hi/uk_news/education/7301354.stm
In 2003, Raj Pandian retrofitted a see-saw with hardware he picked up at the local home improvement store. Check it out at http://findarticles.com/p/articles/mi_qn4200/is_20040119/ai_n10174719 , or take a look at the Prototype: http://www.we-make-money-not-art.com/archives/2005/04/gyms-as-power-g.php
There are also organizations harnassing Play to provide water: http://www.playpumps.org/
And I would reckon a similar system could be adapted to work with swings: has anyone ever seen or heard of that? Are there others saving the world with play?
Aron