Having had a taster of some of the Scottish Islands in 2012 (http://www.alexinthewild.com/p580920295), I wanted to see more and so I flew with a friend for a couple of nights on the Orkney Isles, then alone for a couple of nights on the Shetland Isles in April. Thought had been given to driving up and taking the ferry and this short trip made me hope to return perhaps for a little longer on a third occasion as there is a lot to enjoy, particularly in the scenery, but also historically, especially in Orkney.
Our mission wasn't specifically about seeing wildlife although I did manage to see the male sea eagle in his eyrie on Hoy through a spotting scope. Nothing I could have picked up on my camera as the little group of observers had a hard enough time locating exactly where the eyrie was in the first place. However, it is an exciting development, given this is the first nesting on Hoy in 142 years, and we were lucky to coincide with it. The sea eagles are a young pair mating for the first time, so the egg(s) may or may not hatch.
There is a strong sense of ancient and modern history, particularly on Orkney in a human sense, although Shetland offers an intriguing array of geological history as well as the Norse influence. I was mildly surprised at how very different the two sets of islands were, which underlines that each area should be considered in its own right.
I would particularly have loved to have seen otters, but my timetabling really didn't allow for sitting quietly in wait at dawn and dusk. In some ways, that's a shame, yet although we were very lucky in the main with the weather, it was quite chilly at night, so sitting around at the colder parts of the day might be more advisable in the summer months.
All in all, however, I'd highly recommend visiting both sets of islands: