Nature-wise, Scotland has it all! The islands are stunning—Arran is very easy to access and not too big, but it’s a perfect microcosm of Scotland. Less touristy than others. Mull is great for wildlife, with lots of related activities (like 'safaris,' etc.). Skye is breathtaking but probably the most touristy. Aviemore is ideal for a mix of mountains, lakes, and plenty of attractions nearby (zoo, small amusement park, adventure/mountain activities like quad biking, etc.).
Glencoe is a must-see—the valley is gorgeous, and you can walk relatively flat sections of the West Highland Way. Castles are everywhere.
Any guidebook will tell you what to see. Scotland is quite rural, with only so many roads, so everyone tends to follow a similar route.
In May, the nights are very short, so you’ll need a motorhome with proper blackout curtains—not just those flimsy side-pull ones that let in all the light—or you won’t get any sleep!
J'ai écrit un petit guide d'observation des aurores boréales, basé sur mes propres expériences: https://voyageforum.com/v.f?post=9872650;#9872650