If I had my own personal grassy hill, and few extra thou. on hand... This is what I would buy The Brain for his birthday.
Its called a Zorb.
Like most things that are awesome, (like bungee) the Zorb comes from the magical land of New Zealand. Those dudes are brilliant!
So, you strap yourself and a trusting buddy into one of these things and have someone push you down a hill. These bad boys average around 20 mph.
Remember when you were a kid and you would lay down at the top of a grassy hill and roll down? ... same concept, except inside A BIG BALL! COOL!
You may even get in touch with your inner hamster ;)