Here's how to make a surprisingly sturdy off road go cart using two toddler bicycles salvaged from the local dump, some lengths of wood from a mate's extension and some honest-to-goodness ingenuity. Also requires a hill and a healthy disrespect for your own personal safety. Version 2 did have brakes. This is brilliant fun and is strong enough to take an adult or two.
Full instructions at: http://baddadgreatkid.blogspot.co.uk/2012/09/build-your-own-off-road-go-cart-from.html
Bad Dad website: http://baddadgreatkid.blogspot.co.uk