The Highlands take up a large chunk of Scotland and are about as varied as you'd expect from that. There's the west coast, which is mountainous and dramatic. There's the east coast, which is flatter and more agricultural. There's the Cairngorms plateau in the middle, which is a different kind of landscape again — high, open and genuinely arctic in winter. What ties it together is scale, and the fact that distances between places are often longer than they look on a map.
Fort William is the main town on the western side and a useful base. Ben Nevis is on its doorstep — it's the highest point in Britain at 1,345 metres, and the main path up is straightforward enough that lots of people do it with no particular hillwalking experience, though the weather changes fast and it deserves respect. Around Fort William, the Glenfinnan Viaduct has become very well known through the Harry Potter films and is genuinely good-looking; Neptunes Staircase on the Caledonian Canal nearby is worth a quick stop. The drive towards Mallaig is excellent.
Inverness is the other main hub — the Highland capital, sitting at the top of the Great Glen. It's a functional city rather than a destination in itself, but it's well connected by rail and makes sense as a base for exploring. Loch Ness is just south of it; Drumnadrochit on the loch's western bank is where most of the monster-related activity is concentrated. The loch itself is large and atmospherically dark-watered on an overcast day, though sightings remain unconfirmed.
The Cairngorms is Britain's largest national park and worth more time than most people give it. Aviemore is the main resort town — fine for access but not especially characterful. Kingussie, Newtonmore and Carrbridge nearby are quieter and give a better sense of the area. The plateau walking is serious stuff with proper navigation required; the lower glen walks are more accessible.
Further north it gets emptier. Ullapool on the west coast is a small fishing port with a ferry to the Outer Hebrides and good seafood. Gairloch, Dingwall, Thurso, Wick — towns of varying sizes that serve their communities and give access to increasingly remote coastline. John O'Groats at the top is more of a waypoint than a destination, though the journey to get there is the point.
The North Coast 500 is the driving route that loops around the top of Scotland. It's popular now — perhaps too popular in peak season — but the scenery justifies the reputation. The Kylesku Bridge, roughly halfway along the north coast, is one of the better modern bridges in Scotland and sits in a genuinely striking setting.