My son and I found some videos on YouTube of guys flying down hills on modified 20" trikes with PVC sleeves over the tires. The next day we built him a drift trike.
Codys been building those for awhile now. A lot safer than the riding inside a cardboard box on a skate board thing they were doing...
The trick is to find a good hill that you wont get nearly arrested on for having too much fun...
He recently built a sidehack 2 seater and ran it till the bearings in the harborfreight wheels blew out. He packed the bearingless wheels with grease added fender washers to keep them on, built a ramp, and started jumping it till the frame folded. Lotta smiles, skinned body parts and sore backs that night.