Ed's given you some great advice. I completely agree with you about Aviemore. I just don't see the appeal. That said, Rothiemurchus which is outside of town offers some great walks. I completely agree with staying in Ballater--much better option. There is a wonderful gallery just above Balmoral Castle, the Knock Gallery. Go up for the views even if you don't buy. They often have lovely amber jewelry as well as original art and prints.
You've got a long drive from Oban to Thurso, but I think you know that. Ed's given you one distraction if you're ahead of schedule, I can give you a couple of others. Just past Tain, you'll drive over the Dornoch Firth. It's quite and expanse and quite impressive. On the other side, you could take a detour in to see Dornoch. There is a small cathedral and if you're a golfer there is a famous links style golf course. If you follow directions to the beach you visit the beach and catch a glimpse of the golf course.
If you are way ahead of time and like Castles, you drive right past Dunrobin Castle, the home of the Sutherlands. It has gorgeous gardens and the castle itself is good. If pressed for time don't worry about the museum in the garden, it's lot's of stuffed animals, some other natural artifacts and some other older cultural artifacts, but the gardens and the castle are what are worth the time.
And even if you are late, you can still stop and see the Broch just past Dunrobin. The Broch is on the right, but the parking is on the left. You can see it all in ten minutes and include a walk on the beach with a half hour stop. BTW lock your car as there are often gypsies in the parking lot.