well, in PA (US) we made some big reinforced cardboard armor and attached it to riding mowers (it was at a more or less backyard function, friend's dad owned the land) which was iffy at best, but cheap if you have a mower handy. It was loud and unweildy, but they were fun. eventually we downgraded to tanks where two or more people would climb in and push the cardboard around like armor. It was less of a tank as a mobile firing platform, but it worked. we rigged a potatogun style rpg for a turret. Rules for disabling were varied per game.
Pros: Cheap!!, light, fairly maneuverable and variable. Nobody's tank was anyfaster then anyone elses, they all required teamwork and it could work as a troop carrier.
Cons: totally lame to watch, the guys who were designated to move it didnt have a bunch of fun, and it required levels of communication that arent necessary with machinary. (you dont have to yell at your wheels to turn to their OTHER RIGHT) Also, a powerful gun at close range could give the guys inside a lousy day.