DramMeister, all the best scenery is in the west!
Zulumika, I suggest you start by spending several hours browsing through the Undiscovered Scotland site (http://www.undiscoveredscotland.co.uk
). Now let's narrow it down:
Do you want to visit distilleries? How many? Do you have a favorite?
You want to see castles? Atmospheric ruins, or stately homes?
How interested in history are you? Archeology? There are some amazing sites--stone circles, chambered cairns, etc.
I'm afraid I've given Glasgow short shrift in all my visits, as I find Edinburgh far more attractive, but I'm sure someone here will argue with me. (For your sake, I hope they do.) But I think it worthwhile to start with a couple days in Edinburgh. It's a little tricky, but see if you can get there from the airport in Glasgow without a car. Visit the castle and the Royal Mile, and at least take a run through the museum for an overview of the country's history.
Then head north. There are a lot of attractions in the south, in the Borders and in Dumfries & Galloway, but I think the Highlands are what most people want to see.
You can see a lot in three weeks. The trick is to find a balance between moving and settling. Three nights in any one place gives you two full days to explore it. If you're going to spend a week in one spot, you'll want to be sure that it's a good spot.
B&B's are the way to go in my mind. They offer a good way to meet local people and fellow travelers, although you shouldn't be surprised to find many of them being run by English incomers!
I always push Orkney, because I think it is an awesome place. But if you go there, you will want three full days at the very least.
I don't know about staying in a castle...I can't afford that stuff! But look in the back pages of Scotland Magazine (WM's sister publication) for listings of upscale accommodation.
Here's a twenty-day sample itinerary:
3. Craigellachie (Speyside)
13. Fort William
That's just off the top of my head. Do some research, come back with more questions. You'll have a great trip!