Some comments as well...

HIDDY's suggestion of Stirling is spot on. It's been recently refurbished too, they've done a lot of work on the tapestries, and it's meant to have been done really well.

I'm biased, but I'd also recommend Loch Tay over Loch Ness. Don't get me wrong, the Inverness/Beauly area is beautiful, but it gets busy. Perthshire holds some amazing scenery, some gorgeous little towns and is not so busy as the honey pot which is Loch Ness. And do look in at Blair Castle. It's a proper Scottish baronial style castle, historic home to the Dukes of Atholl. The Atholls had strong links with the 1745 Jacobite uprising (Lord George Murray, Bonnie Prince Charlie's commander was a son of the Duke - Murray is the family name) and have been a major power in the Highlands. Anyway, here's a picture!
There are some nice walks around the Dunkeld/Pitlochry/Blair Atholl area, which don't mean going up high, but let you see some of the scenery.

Can't really comment on the Mull/Skye debate. I can see why Mull is being recommended, but Skye can be breathtaking. Don't underestimate how different the way of life is out on the Scottish islands though. If you are into whisky, Islay might be another option, but its not so scenic. Still very pretty though.

I agree on driving yourself. And outside the central belt, the roads get a lot quieter - albeit narrower too