A week isn't long enough, I spent a month there back in 1979 and only saw a fraction of the antiquities. A viking village was being excavated on the Brough of Bursay (where Skara Brea is) at the time.
Like Noddy says the tomb on Rousay is a must, the Italian Chapel is one of the most beautiful creations, the story goes that back in the sixties the chapel was falling into disrepair and the Orkadians contacted the Italian govt who found the original painter of the fresco who was flown to the Orkneysm touched up his original work and was made an honory Orkadian. It is a most fabulous place and I have to admit to a bit of jealously, it's a place I MUST visit again