A loop around the Scottish Highlands on single-track roads with passing places.
The NC500 loops from Inverness up the east coast, along the north shore and back down the west through Applecross. It's become extremely popular since being branded as a route, which is worth knowing: the roads are the same single-track lanes they always were, and the villages have not grown.
Drive it clockwise, keep the days short, and use passing places properly. The west coast section from Applecross to Ullapool is the reason people come, and it deserves more than a single day.
One car and self-catering cottages are the way to do this affordably — B&Bs on the route price for couples and fill early. Give one person the accommodation bookings and keep them on a shared itinerary; mobile coverage on the west coast is patchy enough that you won't be able to look anything up.