The West Highland Way is the most well-known long-distance trails in Scotland. It's a wonderful trip through everything Scotland has to offer, starting off in the beautiful rolling hills of Milngavie near Glasgow, continuing north past Loch Lomond, and ending in Fort William just past the very first mountains of the highlands.
It's a uniquely social trip. Since the trail is so popular, towns have sprung up everywhere that people end the day; with these logical places to spend the night, you naturally end up roughly following the pace of those around you. With how friendly everyone is, striking up a conversation on one of the first days is guaranteed to get you a walking partner for however long you wish.
It's an incredible experience and one of my favorite backpacking trips. The trail itself is extremely beginner-friendly, with hostels and baggage transfer available if that's your thing. I would wholeheartedly recommend it for anyone even remotely interested in hiking.
The Cairngorms National Park consists of a small mountain range nestled between the towns of Aviemore and Braemar. The area is excellent for rock and ice climbing, and is home to a significant mountaineering community - but also features a couple of trails around and across the central mountains. The trek around the mountains starts off relatively simply, following the western edge of the mountain range. Starting in Aviemore, the trail passes through woodland for the first day or so, and then leaves civilization behind as it mirrors the movements of River Feshie.
After approaching the southern side of the mountains, the way back goes through the central mountain pass, Lairig Ghru. This makes for some slightly more challenging hiking, but nothing that can't easily be completed within a day.
The area is seldom used for backpacking, with most activity being related to skiing, rock climbing or mountain biking. If you are looking to backpack in Scotland, I would recommend picking the highlands or West Highland Way over the Cairngorms; but it is nonetheless a relaxing 3-4 day hike with decently varying nature.