A great project for Webelos Forester Activity Pin.
A tree cookie is a one inch thick slice of a tree branch, about three inches in diameter. Select a green branch that has already been downed - do not harm living trees, especially trees that you do not own. After slicing the green cookie, roughly 30% of their weight is water, which will need to be removed.
Dry the cookie by leaving it in a cool, dry place for a month or so, or place in the microwave on high for a few minutes. Be sure to open the microwave every couple minutes to release the build up of steam and to flip the cookies over once in a while. When they no longer steam, they are done. Do not over-'cook' the cookies, they can start to char. Allow cookies to cool completely.
After the cookies have been dried and cooled, sand both sides. Begin with a very rough sandpaper, 40-60 grit, move to 100 grit, and finish with a 200 grit sandpaper. Belt sanders or random orbital sanders work well to speed up the process.
After sanding, you will need to varnish or polyurethane or both sides. Spray, brush or dip the cookies. Multiple coats may be needed to better protect the wood. Spray dries the quickest, and has the least cleanup. Allow to dry overnight and the cookies will last for many, many years.
Remember to keep Safety first, even a hand-saw can cause a nasty wounds. Power tools should only be used with adult supervision and always wear safety glasses.