I am off to Scotland in May.
Any landscape spots I should not miss?
I Lived in St. Andrews for a year during undergrad, and I got around a bit of the country, from Dumfries to Orkney; I haven't spent much time in the islands, but in the West I would very much recommend Glencoe and Isle of Skye.
Glencoe has great steep hills which highlight the classic glen, given some mist (which I had). On its own, it may be a little bare, and it may be best suited for serving as a backdrop for individual subjects. You will likely find this is a common problem in much of the area you are planning to go to, as the highlands are hills and heather and precious little else in many places. Finding interest will be the challenge in many places.
The Isle of Skye, due to its healthy amount of rain, is absolutely covered with running water. Unless you are cursed with a dry visit or weather so wet that you cannot actually compose, there should be waterfalls all over the place. The high places of that island are beautiful, and likely hold some great shots.
In Edinburgh, climb Arthur's Seat and look out in all directions for some interesting takes on the landscape and skyline; if you can find it, there is a great little guidebook sold by Royal Scotland in their Holyrood shop. There is a park in a valley alongside the Royal Mile (High Street) which provides some interesting possibilities for incorporating the valley and the castle into the rest of the city (or cutting it out entirely). The necropolitic monuments on the hills facing the royal mile can provide interesting contrast with the classic city of High Street, and wandering some of the smaller paths around the city core can provide some vistas of the Royal Mile itself.
I did not go down to Holy Isle (in the borders), but if you can make it down there, the proffile of Lindisfarne is striking--I particularly like the way Lee Frost has captured it.
If conditions are spring-like, I would recommend New Abbey, a small town outside of Dumfries. Now, this may be out of your way, as it is South of where you are going, but distances are small in Scotland, and the day I was there in late March, it was the textbook definition of pastoral idyll: ruined abbey (Sweetheart Abbey), fields of cows, forested hills, spring flowers, a rushing stream. I very much liked the area around Dumfries; if you are looking for a more barren look, there is Threave Castle nearby, and Caerlaverock Castle is nice, though I am not sure there are any good angles to put it into a landscape.
Outside of Stirling, do consider [a href=\"http://www.undiscoveredscotland.co.uk/dollar/castlecampbell/]Castle Gloom[/url] (Castle Campbell), which is situated on a hillside overlooking a steeply-descending stream ("the burn of sorrow"). It probably is best with fall color and I am not sure what angle you can get on the castle itself (my visit was between SLRs and I was a tourist, not a photographer), but I would look into it, even if you eventually decide against it.
I could make a variety more suggestions, but I am not sure what aspects of the Scottish lanscape you are particularly interested in capturing, and therefore will restrain myself (if the above could be called restrained).