I spent almost 20 days in Scotland but if you only have (basically) one day I suggest you visit Edinburgh (if you’re into museums and City Sightseeing), another place I really loved is Inverness, a bit farther away but you can fly there from London with easyjet. It’s a really small town but lovely, and the surroundings are just amazing. Other place I’d go (but it’s super smallish) is Oban!