There seem to be a lot of duplicates of "hidden" gems!
For what it's worth, mine are:
The drive down Glen Etive near Glencoe. (Note it's not spelt Glen Coe)
The Ardnamurchan Peninsula via the Corran ferry.
Altnaharra on Loch Naver for truly breathtaking, remote scenery
The Cairngorms - great for Ospreys and wildlife
The Border towns have a fascinating history
Trumpan at the northern end of Skye
Orkney (not Orkneys) but the ferry crossing can be hideous if like me you are a poor sailor
Aviemore - incongruous design - a bit like a cheesy, poor man's version of a Disney town surrounded by great countryside - stay in the much nicer nearby Boat of Garten or Nethy Bridge - so much classier and traditional.
Loch Ness - hideous traffic and full of tourist tat everything (hotels, Ye Olde Tea Shoppe etc). Suitable for children only.
John O' Groats - not much there. Bleak.