For what it's worth, we did the drive from Inverness to Edinburgh via Balmoral and Glamis in June. We left Inverness around 10:00 a.m. and arrived in Edinburgh around 6:00 p.m. So, I'd say that you'd be hard pressed to do more than two sites along the way, if you're taking the Eastern route. An overnight along the way would allow you to see a lot more.

As an update, we are now staying three nights in Grantown-on-Spey instead, giving us the three days to explore the castles at our leisure. Well, two days - one day will be spent in Inverness getting Gaelic language, history and music lessons from a lovely lady named Margaret :) (complete with ceilidh at night!)

That will be nice !