Build Your Own Battlemech
Jodrell writes "Hot on the heels of the Back Yard Millenium Falcon comes this: a BattleMech Tree House. Using a couple of packing crates and some angle iron, a very dedicated Dad built a tree house around a Madcat 'mech (or a Timberwolf to those in the know) that stands about 25 feet tall. I want one :-)" And in a related story, BoomZilla points out that you can pick up a Rebel Blockade Runner on Ebay. Of course.
Depending on how old they are (and assuming you have space available) tell them "Ok, as long as you help". Obviously heavy metalwork is a bit risky, but they can do things like cut wood, paint, or screw things together.
This way, they get a 'Mech, you get a 'Mech, you find a productive use for youghful energy, and they learn useful DIY skills that will come in handy during adult life.
Maran
Some company in finland figured out a way to build a forest harvesting machine that walks.
It uses a diesel engine driving a hydraulic pump to acutate it's limbs. Check it out:
http://www.plustech.fi/Walking1.html