What trim can I use on wooden stairs?
I personally think adding trim will just draw more attention to the side of the stairs.  And no matter now nicely you trim it, it will stand out as people are not used to seeing trim on stairs.

In my opinion, I would just use caulk to fill the gap between the existing skirt and treads.  The gap looks pretty small and most people will never realize what you've done.

Edit:
I just realized...  There is no skirt board in those pictures.  You can add a skirt board after the fact but it is a very time consuming task.  See the link for an idea of how to do it.  http://www.finehomebuilding.com/how-to/tips/scribing-a-skirtboard-to-an-existing-stair.aspx