Hey there Sfan.....re good driving roads.
When you're in the Trossachs area presumably somewhere near Stirling, take the A84 road to Callander ( a lovely little beauty spot around 15 miles or so outside Stirling )...If you can, follow on from there and take the A85 which becomes the A82 road onwards as sign posted for Fort Willaim. This'll take you up through some fantastic roads ( rally style twists n turns through mountain passes and Glens ) but you'll be so impressed with the scenery you'll probably dawdle along. You'll pass Bridge of Orchy and follow on to Rannoch Moor
and eventually reach Glen Coe.
Allegedly the Campbells murdered the McDonalds there in around 1692 but its all tosh trust me.....I'm a Campbell, I know its all lies...lol
After you pass the Kings Hotel and the ski resort if you look very very carefully on your left hand side as the road gets a little tight and you pass through some blasted rock you will see ( if you're lucky ) a stunning waterfall. If you miss it, but go as far as Glen Coe you can't miss it on the way back"
Its absolutely gorgeous, and if you're lucky and get a really nice / hot day there is a layby right next to it where you can stop ( as most people do ) it makes a great photo opportunity, or better still, either for you and or the kids, a great cooling off point.
I've jumped in myself more than a few times ( its great fun ) and the water is crystal clear and ice cold. If you climb up to the top of the waterfall, there's a "sink hole" around 6 foot dia and around 15 foot deep.......try jumping in there....lol
As for Ballater - you can either take the A93 from Blairgowrie right up past Balmoral Castel and Crathie to Ballater which is a beautiful run OR head up the A94 for Aberdeen and cut off at the B966 for Edzell and then onto the B974 up and over the "Cairn O Mount" to Banchory, take a left onto the A93 and back into Ballater that way. This way you get some faster dual carriageways, a shortcut and some great scenery too.
Hope you have a great holiday
All the best
Jim