We were there for a couple weeks last May so I'm still pretty fresh on the details!
If you are going anywhere near Edinburgh, and haven't done so already, you need to book last week. The Fringe and Tattoo both happen in August (last for most of the month) and any and all lodging will be booked.
US driver's license works fine. The roads are curvy, narrow, and they drive fast so expect to be passed often. We stuck with DH driving as we were told it was easier for one person to adapt than two. I did drive down Glen Etive (one lane road used in Skyfall) but I stuck to navigating the rest of the time. Roundabouts are very common -DH found it helpful if I'd give him the number of the exit instead of "going to the right, going straight through", etc. 1st, 2nd, 3rd exit was more helpful to him. Make sure the navigator helps the driver be in the correct lane prior to entering the roundabout or you will be honked at if you try to switch lanes mid-round. Some of these roundabouts will have like 3-4 lanes and stop lights!
We stayed in Edinburgh first and waited until we were leaving before renting the car. You won't want to drive in Edinburgh/Glasgow. Edinburgh has a cheap and efficient bus system so if you stay somewhat near City Center, you'll be fine.
The Glencoe area is absolutely, stunningly beautiful. Probably my favorite place I've ever been. We were there three days and I absolutely plan to go back some day. Lots of hikes to do if that's your thing. Happy to recommend specific ones. Distillery tour was worth the time (we did the one in Oban - if you go further north I'd probably try ones up there as they are more well-known) and can give restaurant and B&B recs if wanted. Scottish breakfasts were probably my husband's favorite part of the trip. It also saves you money on lunches because it fills you up!
We did Loch Lomond for a couple days and a combo of rainy weather messing up hiking views + their Bank holiday made the area seem overly touristy to me. I'm not sure I'd go back there. It was probably worth visiting (and hikes on clear days would be gorgeous), but I'd rather go back to Glencoe or further north.
Isle of Skye, Mull, and others are extremely popular. We didn't have time to go that far north as we spent time in England as well, but if you have 10 days solely in Scotland, you should be able to at least do Skye. The ferry to Mull runs out of Oban which again, was a neat town to visit.
Happy to give any help or advice - the time spent in Scotland was my favorite trip we've done!
