Drop in on Benbecula on the way or the way back, or even instead of..
Stunning scenery and far more civilised than Stornoway.
Que?! Have you ever actually been there? You must be the only person I've ever come across who thinks BEB is "more civilised" than SYY... there really isn't very much at all there and as mentioned the security experience is, er, interesting.
Only thing I'd say to the OP is make sure you take a tour round Harris while you're up there. Best scenery in the Western Isles by a long shot and the best beaches in the country.