Respect for the effort, and God I wish I'd been with you (Sitting in a Travelodge in Slough as I type!) but you have done Lewis a huge disservice.
There is loads to see, dozens of megalithic/prehistoric sites, all the history of the last 2000 years, the wild full-on scenery, the blasted moonscape of Harris' east coast, the flawless beaches, massive skies, dozens of artist's studios & hideaways displaying & selling all sorts of things inspired by, & produced on the Island. You can talk to these people for hours, and their personal stories are all interesting & uplifting.
You can visit Callanish at different times of day and see 4 different sites.....
Your trip was brilliant, and you've seen the best, but you did miss a lot - Lewis is a place to be savoured.
Still jealous tho ;-)