Xylophones were probably one of peoples' first musical instruments, and the feel of playing them is most firmly entrenched in our primal core. I've seen babies barely able to pick up mallets know just what to do with them.
Just about any one can make a xylophone out of just about anything- i.e. old shoes, cut up tires, bones, V.W. hoods, etc.... (official definition has a xylophone made of wood, so I guess we'd have to call them shodaphone, blimpaphone, bonaphone, volkphone, and et ceteraphone.) The main trick is just mounting whatever it is in a way that it will vibrate minimally hampered.
For the sake of simplicity, I'll describe the how-tos of building a wood xylophone, but the same techniques could be applied to any material. These instructions are intentionaly very general to promote creativity. For more detailed info there's some great books on basic instrument making.
First you take a chunk of wood (straight, clear, dense grain is ideal, but I've gotten neat sounds out of pretty gnarly looking stuff) and cut it to a length that suits. Too long or too short won't resonate very well, so experiment. Now, find the nodes by either measuring 22.5% of the length in from the ends or sprinkling some salt along the top and tapping it lightly- the salt will collect at the nodes. Support the wood under the nodes with something soft, like felt, foam, or balled up socks. Tune it by cutting it shorter to heighten the tone, or gouging under the middle to lower the tone. You can also raise the pitch slightly by thinning the ends, and lower the pitch by making a simple saw slice or gouge in the middle. The ideal shape for a xylophone bar is something like this:
This general shape gives you the clearest deep tones, but shaping it thus can be a lot of work and isn't necessary to get a decent sound out of any but the very low tones.
To check out possible tuning options check out "tunings" page.
After you're done tuning all the keys, you'll want to re-figure all the nodes and mount the keys at those points. If you gouged a lot under some keys, use the salt method to find the nodes, because gouging moves the nodes outward. As long as the keys are supported by something soft and are not bound in a way as to restrict their vibration, mounting possibilities are limited only to the scope of one's imagination. The most commom methods are: drilling an over-sized hole at the nodes with a nail or peg holding the key in place, or wrapping twine around the key at the nodes and fastening the twine.
As for mallets: hard mallets tend to bring out the high end and soft mallets the low. Super-balls, rubber-band balls, etc. work well for soft mallets, and there's lot's of options for harder mallets.
Imagination is our best resource. Go with it!