I spend 3-4 months in Scotland each year. You will be there just before the weather turns a bit nasty for the season.

No idea what you enjoy, but since you say you'll be going alone, I'm thinking solitude is something you are after? If you are feeling the least bit adventurous, drive up to Loch Lomond and rent a small rowboat from any number of establishments on the shores. Loch Lomond has more islands than any other Loch in Scotland, many of which are believed to be man-made and dating back to pre-historic times. At any rate, row out to one (easy to do) and you'll find yourself on a moss-covered piece of land with tall trees and not a soul around. Take a picnic lunch and a bottle of wine. Very nice way to enjoy the day.