The A9 down from inverness to perth is crap - it would be ok if it wasn't so busy or if you're making a late night/early morning run and only have a few things to overtake in one go! But at normal times you'll be stuck behind lorrys and buses for ages (Admittedly I have pulled off my most impressive overtake ever there but you need to know where to do it, plus there are a lot of accidents on the A9 and hence unmarked 5-0, they even use to patrol in impreza's and goad the unsuspecting into races!) Anyway...
The old military road would be a good fun detour option - it's just been featured in evo magazine where they consider it to be the best continuous stretch of drivers road in the UK, so high praise indeed. You can start on that road from Inverness and avoid the A9 almost entirely - head across to Nairn then down or cut off the A9 through Carrbridge then onto Grantown on spey, head all the way down to Blairgowrie (via two ski "resorts") then head onto Perth from there.