Go to your local hardware store (NOT Home Depot, ACE, etc.) and ask if they have any lumber or crates or anthing lying around that they would give away for free or for very cheap. (unless you want to spend some cash) A lot of times they have pallets lying around that they want to get rid of. Pick up some of those, a hammer, some nails, and then just use your imaginatoin.
I found an old long wooden crate in our barn (it was about 2.5 ft high, 3 ft wide, and 8-10 ft long) and I layed it out on the grass at the edge of a small hill with a ramp up one side. I rode the ramp up, and jumped off the edge onto the hill. When I got good at that, I removed the ramp and tried hopping up onto the crate andjumping off the other end, which proved to be a lot more difficult.
Basically just use your imagination. You don't need to have 20 different stunts set up to have fun.
edit: cinder blocks can be useful too.