St Magnus Cathedral, founded by the Vikings; the Italian Chapel, built in a Nissen hut by PoWs; the lighthouse & visitor centre, North Ronaldsay; Scapa Flow visitor centre; the museums in Kirkwall & Stromness; Barnhouse Neolithic village; the Pier Arts Centre in Stromness, and that's just for starters!

I'd add the ring of Brodgar to that list

http://www.historic-scotland.gov.uk/propertyresults/propertydetail.htm?PropID=PL_233

And also Maes Howe

http://www.historic-scotland.gov.uk/propertyresults/propertyoverview.htm?PropID=PL_205

SM