I especially liked a trick that caught many amateur railgunners - the projectile should be injected into its rails with some initial speed. Otherwise it will just melt inside.
But the best thing I finally found was this highly sophisticated gauss rifle:
The site "Science Toys You Can Make With Your Kids" is quite good - many things from it can actually be created with kids. Some of them look dangerous, but only for those who doesn't know what kids usually do when you turn your back to them.
My favorite is Film Can Cannon It's a real pity that digital photography will soon make it obsolete :)