Been twice for 2 weeks. First time, 1 week Kirkwall, 1 week Papa Westray (left the car at Mainland airport 'cos Papay is totally walkable). 2nd time 1 week cottage north of Kirkwall, 1 week Sanday, car ferry.
If you want solitude + walking in landscape, both Papay + Sanday are WONDERFUL.
On Mainland stay, we preferred Kirkwall for overall flexibilty. Strongly endorse a visit to Banks at South Ronaldsay (excellent cafe/restaurant). Totally different experience to the nearby Tomb of the Eagles.