When I went to Scotland in March we traveled from Edinburgh up to Inverness to Elgin. Made the mistake of renting a car. I would second the notion of relying on trains. As one Scottish gentleman said - why drive when you can sip whiskey on the train, talk to people and see the scenery go by? A lot of castles as well as whiskey tasting places are closed in the winter. But the major ones are open, such as Sterling as one person said. I recommend seeing the cathedral in Elgin. Elgin also has a lovely (but expensive) wool factory. Roslyn Cathedral is lovely as well. It's only 20 min. south of Edinburgh. Oh, you can also sign up for bus tours from Edinburgh to major spots in the highlands etc. Just go to the big tourist place by the river.
Have a great time!! I loved Scotland and look forward to going back!!
Savra
2006 Specialized Dolce Elite/Specialized Stock Saddle
2011 Surly LHT/Brooks S Flyer