What a great birthday present.
I liked Callanish, who wouldn't. It's epic. But I think I liked this better
https://www.themodernantiquarian.com/site/199/ceann_hulavig.html
because you could have it to yourself. It's just down the road.
Also this is superb, and you will be glad if you go
https://www.themodernantiquarian.com/site/1277/bostadh.html
https://www.visitouterhebrides.co.uk/see-and-do/bosta-bostadh-iron-age-house-p523981