Ullapool area is stunning. World class in my opinion, complete wilderness nearby, fabulous beaches, fantastic hill walking.
Haven't been for a few years sadly as no longer in the UK, but when I lived in Scotland (on the east coast!), that was always first choice for a venue. It's 4 hours (fast) drive up the A9 from Edinburgh.
Other parts of the West Coast also spectacular. The East has it's charms, and the weather is often better. Both Edinburgh and Glasgow worth a visit, but depends on what you like to do on your holidays. Edinburgh is an immensely beautiful city and lots of culture but will be packed in August. Glasgow very vibrant, if you do go, don't miss the Burrell Collection.
Fife is nice, but not especially different from much of rural England in terms of scenery etc.
Agree that wildlife park at Aviemore is worth a visit, especially with young DCs, but while I love the Cairngorms, it's pretty bleak in Summer (outside of Rothiemurchus Forest) and you really have to like your wilderness walking to get the most out of it in August.
Inner and Outer Hebrides also stunningly beautiful (ferry to Lewis leaves from Ullapool), plus (apart from Skye), the rain tends to pass over the islands relatively quickly, so you often get brief showers rather than days and days and days of rain. If you're into sailing, how about renting a boat for a few days, there is a place on Skye at Armadale which has some of the finest scenery in Europe on it's doorstep.
It depends a bit whether you are intending on touring around or staying in one place?