St. Andrew’s to fort augustus

What do you mean when your say 'interesting'?

Different people have very different perceptives... Some find shopping intersting, others may prefer scenery, walking, museums, whisky, Loch Ness monster...

I would look at the route through Pitlochry and then towards the Great Glen. There are some good stops on the way. I would pick two or three and leave enough time for Fort Augustus as well. If you like scenery, history or whisky, you will have a few good options on this route.