I just did a long day road trip from inverness, up the East Coast to Wick and on to John Groats. (This road is all two lane, oh my there are some steep hairpin curves.) I did the short hike over to see the Duncansby Stacks. Even this late the number of birds were amazing. I then headed west along to Thurso and then took a wee road down to Helmsdale. It was single track, but I think that I saw at most ten cars the entire 38 miles. It was gorgeous. You would have a another hour on the trip from Aviemore.
If you're wiling to drive south think about just heading out to Rannoch Station from Queens View. It's a beautiful drive. This is technically the road to the Isles. When I was out there late last week, the winds of Hector were pouring over Rannoch Moor and whipping up the surf on Loch Rannoch.
A word of warning about picking up your car in Glasgow. If you are picking it up city centre, you may run into the same long wait that I had at Edinburgh. I was supposed to pick it up at 10:30. i think stood in line for 45 minutes as everyone dithered over insurance. They had three staff members, but each person was 10-15 minutes to serve. So, get there early or take a bus out to the airport to get the car.
When you are in Pitlochry, check out the Explorer's Garden. Beautiful place with some interesting history. On the way south there is the Highland Folk Museum in Newton More. And I enjoyed a quick stop at the Ruthven Barracks. If you take the wee road instead of the A9 you'll drive right past it.
Back in Aviemore check out the Rothiemurchus Estate for some great hiking.
Single track roads are not that bad once you get used to them. In fact, I kind of like them better. You don't have to pretend that both cars can fit on the road like you do on some supposed double lane roads!