Some wood, would work,
just don't use treated or painted wood.
I think, though if you are intending
to use the charcoal for water filtration...
I would recommend starting with something edible like corn, wheat, or rice, though, and turning that into charcoal. I have also made activated charcoal on an electric stove, but you really don't
want to do this indoors
, for the same reason that you don't burn a charcoal barbecue inside
, as you could produce carbon-monoxide which is very toxic.
The heat breaks apart the wood/grain carbohydrate molecule and drives off everything below the sublimation point of the carbon
leaving just and only the carbon.
Carbon also has the highest melting and sublimation point of all elements. At atmospheric pressure it has no actual melting point as its triple point is at 10 MPa (100 bar) so it sublimates above 4000 K. http://en.wikipedia.org/wiki/Carbon
When the flame from the hole in the top of the can goes out, it's ready, but careful, the carbon will start to burn, with oxygen, and you will be left with nothing.
A simple charcoal filter can be made with a vertical length of hose, with a nail through the bottom end, to stop the charcoal chunks falling out. put a funnel at the top and drain your water, or whatever liquid, through.
You may want to add a little fine metallic silver, to prevent the growth of bacteria in your charcoal filter, if you are going to reuse it.
Like I said in my first post,
do this at your own risk.