There's not much to do there, some sightseeing, primarily the Callanish standing stones. Stornoway is a pretty town. I took a bus from Stormoway to Harris, saw fields full of farmers taking turf (for the fireplace) out of the ground.
The Wee Free Kirk of Scotland pretty much rules the place, meaning that on Sundays, the swings in the children's park are locked up tight so the kids can't indulge themselves. Needless to say, I didn't spend Sunday there.
It's worth it if you like quiet and beautiful scenery.