Churches, Chapels, Monasteries, Priories
Scotland has hundreds of ruined monasteries, abbeys, priories, friaries. Whatever you call them, these religious establishments housed lay brothers and clergy living separated from the rest of "the world"
Add to the roster of monastic installation all of the parish churches, chapels, and cathedrals scattered over the country (I have a book listing over a 1000 churches to visit) and you could easily make a trip out of seeing nothng but religious ruins.
Abbeys and Churches in Scotland
The biggies, of course, are the Border Abbeys -- Melrose, Jedburgh, Dryburgh (and Kelso), but some of the others are beautiful. I particularly like Abroath and St. Magnus Cathedral
There are abbey ruins from many of the major monastic orders -- Fransciscan, Benedictine, CIcstercian, and a few smaller ones. It is interesting to see the differences in the layout of the buidlings and their design between the orders: tall towers vs squat towers, plain vs ornate. After awhile, it's possible to figure out which sect built the place just by the blueprint and remaining ruins.
Another good resource is the Abbey Explorer's Guide, a great reference book about the bits and bobs that make up a monastic site.