Biggest problem in the summer on either of the two main routes North to the Highlands A9 & A82 are just horrible - getting frustrated behind caravans is almost certainty.
From Forth Bridge, and Dunfermline, take A823 then A822 to Crieff;
From there either left on A85 to A82 on up to Ft William or Right then A822 again up through Amulree to Dunkel if you want to get back to A9
The A86 from Spean Bridge to Kingussie that crosses from the A82 Ft William to Inverness over to the A9 to Inverness is a nice road - it's not challenging but really pretty road and tends to be quite quiet even in summer.
Over to Skye via A87 (off the A82) should definitely be on your itinerary