Those who follow Steve Marsh will have seen his new video about the Isle of Hoy, Orkney- https://www.youtube.com/watch?v=-6vrFfIW1es
This is a different part of the Orkney experience, which it seems few if any people on this forum get to.
To clarify matters a bit from the video there is a lot else to do on Hoy- all of which is on this website- https://hoyorkney.com/travel/travel-to-and-on-hoy/
That includes a lot of important WW2 sites, especially the Scapa Flow Museum, which is as much of a must visit as the Italian Chapel and the Churchill Barriers.
As well as the little ferry from Stromness to Moaness shown in the video there is also the vehicle ferry from Houton to Lyness (for the museum) and Longhope. You will also see from the website that there is a pretty good Island bus service. I have visited the island in the past (in fact stayed at Longhope once) and pretty comprehensively visited everywhere using the bus service, so a car is far from an essential. Buses also link Kirkwall with Houton Ferry Terminal very comprehensively.
This is more about Graemsay, mentioned briefly on the video- another lovely and almost car free island to visit (as I have)- http://www.graemsay.org.uk/
A half day trip from Stromness.
Also, in between Lyness and Houton is another little visited island- Flotta-https://islands.scot/island/flotta/
It also has a significant war time history, although the island is now better known for it's oil industry. I've been there several times, without a car. The island is eminently walkable.