Going to Scotland in March - Advice??

Usually we head to sunny St. John USVI for spring break but this year we're up for an entirely different kind of vacation - we're going to Iceland and Scotland.  Do you have any advice for us?  (myself, Dan and our 9th grader)

We'll be landing in Iceland on a Sat. 6:30 am  for a one day/night visit and then we'll leave Sunday am, arriving into Glasgow by 10 am.  We pick up a rental car and head to Edinburgh where we'll be staying for three nights in a 2 bedroom flat located on the Royal Mile.  We'll leave Edinburgh Wednesday am and don't have to be back to Glasgow for our departing flight until Sunday afternoon (5 days/4 nights).  I'm a huge fan of Aspiring Kennedy and have made notes of her suggestions for Stirling, St. Andrews, Portree and the little village of Luss.   My friend Anne also recommended Oban.  We definitely want to head up to the Highlands and don't mind the idea of inclement weather, in some ways I find myself hoping it will even snow!

Is there a grand country manor or castle you stayed in that you'd recommend?  Any sights we shouldn't miss?  Thanks for any and all ideas.  Michele xo