Kate of Minieco has made her own threading beads from a branch in her backyard. As long as it isn't rotten, it should cut just fine with a small saw. Drill holes through the middle of each piece and you're all set.
To keep things interesting you can have different colors of string (or shoelaces) available for threading or even different lengths. For more quick crafts check out Kate's recent printable sewing cards. Between those and the beads, you'll be all set for hours worth of entertainment! Check out the full tutorial on making your own threading beads (or ones for friends as gifts) over at Minieco.