Ooh, I got one! Where I used to live, we had a couple of old disused watertanks (open-topped) in the backyard, as well as one still in use. One was used for storing firewood, and the other had just been sitting there empty for some time.
I had a friend visiting, and we had the bright idea of climbing into the empty one and running in it, doing an impersonation of rats in a wheel. So we're speeding back and forth across the yard, propelling this tank along with our feet, falling over repeatedly as we flattened various plants, knocking shit over and battering the chook pen, until my older brother told us to stop.
Then we sat around for a while, arguing over what to do, until he went out. Then it was on, this time we were serious! We had just decided to try and steer it out of the backyard and see how far we could go when we SLAMMED into a large tree, bouncing up and down and rattling around inside. Then the rusty old metal started to buckle in the middle, which could have ended very badly... Luckily my friend was rather taller and stronger than the young me, and held the thing up as I made my escape before diving out himself as the whole thing collapsed and fell over noisily.
Another time, a few years on with the other friend I mentioned, the one with the broken arms, (Edit: Err, he didn't have broken arms at the time, obviously...
) we wanted to build a go-kart to celebrate our love of rolling down hills on dangerous contraptions. Unfortunately, our grand plans involving roll-cages, converted shopping trolleys and pedal systems sorta hit a wall with the realization we had no welder, nor the money to hire one.
So, we settled for just getting an old, battered pram, adding a wooden plank to sit on, and then rolling down the BIGGEST DAMN HILL WE COULD FIND.
The thing had no steering whatsoever, and the brake was a wooden fencepost jammed in the side. My friend and I took turns to sit on the kart, whilst the other rolled down the hill behind him on a cheap wooden skateboard, the back end of it broken off from previous adventures, and kicked the wheels of the pram to 'steer' it.
Well, it was fun, I must say! But as we started to gain speed, bad things happened... At first, not going too fast, I was on the kart when we crashed over a small curb and into a garden. I was rather proud of my mid-air acrobatics, diving out of the thing and to one side as it smashed into a tree.
Then, a bit later and on a MUCH LARGER HILL, I was on the skateboard on steering duty and my buddy was on the kart/pram. We were gaining speed pretty fast, and I was desperately kicking the kart's wheels to try and steer it around a long corner at the bottom of the tree... But the shitty skateboard was going waaay too fast, and started fishtailing all over the street. I managed to control it for a while, steering the thing with my HANDS (skinning the palms in the process!) whilst still keeping up the steering, but eventually the thing just shot out from under me, I crashed to the ground and could only watch in horror as my friend continues on down this rather large, steep hill...
I fully expected him to get horribly injured, especially with his track record, but somehow, with ABSOLUTELY NO STEERING he just happened to be going in a straight line and hit a driveway rather than the high curb at the bottom of the slope. He went up, over a nature strip and then back down another concrete ramp, completely unharmed. Amusingly, I was the worse injured of the two.