It all depends on your tastes of course. One of my favorites is http://www.tesnexus.com/downloads/file.php?id=8261 (An age retexture mod which adds variety to all the NPCs, as well as to your own character.)
One of the most impacting changes to eyes, IMO, is adding eyelashes. There are a few eyelash mods out there, the one I'm using is http://www.tesnexus.com/downloads/file.php?id=24085.