Try the west highland way in one sitting.
There'll be lots of people telling you there's better riding to be had elsewhere etc etc and they wouldn't necessarily be wrong, but i'd say about 80% of the riding along the WHW is great, either technically or scenically.
Plus its an iconic route, easily followed, its easy to get to and get back from, there's accomodation at either end and lots on the way if it goes wrong, people to chat to along the way etc. If you wanted to include some bothying or camping you could stop off at Kinlochleven and extend your trip up to Inverness via the great glen way, although what i rode of the GGW was pretty tame.
Do it in may or june, expect it to eat your bike, and give yourself at least 15 hours or so (starting in the small hours at Milngavie helps).