Highland (Scotland): See
Castles for the most part are sternly defensive but tumbledown. Great examples are Eilean Donan at Dornie and Urquhart Castle at Drumnadrochit by Loch Ness. Luxury castles (Victorian gin-palaces with twiddly bits to make them look medieval) are Dunrobin Castle at Golspie and Castle of Mey near John o'Groats, plus the Georgian Inveraray. McCaig's Tower above Oban is a Victorian folly.
Gardens: every west coast promenade is studded with palm trees shivering in the rain to make a point about the balmy Gulf Stream. What grows better is Himalayan species, and you know you're within five miles of a stately mansion when you encounter their escaped rhododendrons, a riot of colour in May. The standout gardens are Inverewe near Gairloch, Arduaine near Oban and Crarae near Inveraray.
Deserted landscapes: the rural Highland population was once greater, but they left or were driven out. Haunting Glencoe was a 17th-century example, and Auchendrain near Inveraray depicts farmhouse life before the big exodus of the 18th and 19th centuries. On several hillsides you see abandoned villages, and at Helmsdale the reverse: the makeshift village erected by those evicted from their crofts, before they despaired and headed to the lowland cities.
Prehistoric structures have mostly been obliterated by agriculture, the best are in the Hebrides and Orkneys, but there's a fine collection around Wick. Most common were "Duns" - fortified outcrops, often on the coast. They're scrappy and you mostly come for the sea view.
Islands involving a ferry ride are described elsewhere as part of the Hebrides, Orkneys or Shetland, but The Summer Isles are an example of an inshore archipelago, reached by boat trip from Achiltibuie. Unique within the UK are the "mainland islands", settlements and drivable lanes that have no road connection to the mainland network. The most populated is Knoydart, reached by ferry from Mallaig or by a very long hike across the moors. Several others such as Applecross were only connected in the late 20th century.