if the crack is only on one type of wood, get some sawdust from that kind of wood (Get a scrap and use some real coarse grit on it), then take the dust, mix with wood glue (Nothing special, it wont have any tension on it) until its kind of pasty. force the mix into the crack and rub around, leave excess on. alow 24 or so hours to dry, then sand smoothe. if the crack is on several types of wood, you might could get creative with different types of dust, but it would be harder.
good luck