Haggis is a food strongly associated with Scotland and I can assure you, I've never eaten it nor am I going to start now. Here's the official Wikipedia description: it is comprised of sheep's 'pluck' (heart, liver and lungs), minced with onion, oatmeal, suet, spices, and salt, mixed with stock, and traditionally simmered in the animal's stomach for approximately three hours. Most modern commercial haggis is prepared in a casing rather than an actual stomach. Blech!!
In addition to haggis, you might find cock-a-leekie soup (chicken soup with leeks), Scotch pie (think pot pie), Neeps and Tatties (mashed turnips and potatoes), and Rumbledethumps (potatoes, cabbage and onions) on a typical Scottish restaurant menu. They don't eat as much beef as Americans do but still eat a lot of pork and lamb. In Edinburgh and other coastal cities, you will find north Atlantic seafood (salmon, in particular) on most menus.
I visited Scotland in 1988 while I was studying abroad and I remember Scotland being well-known for (or at least well-stocked) with sweets - particularly shortbread biscuits (cookies), vanilla fudge and toffee. You'll find some of the same English sweets in Scotland (like scones, McVitie's biscuits, trifle) but each region has its own idiosyncracies.
By the way, this made me think of something else worth mentioning about the differences between the English and Scottish. Over here, we tend to use the terms British and English interchangably but the United Kingdom is a sovereign state made up of the island of Great Britain (Wales, England and Scotland) and Northern Ireland. In Great Britain, a native person will probably introduce themselves as Welsh, Scottish, or English first and then identify themselves as British. Make sense? All Scots are British but not all Brits are Scottish.
We will definitely see two out of the three (BP and P of H) on our trip - no matter which vacation we choose and we may be able to see the third castle on an optional excursion. There are certain must-see sites included in every vacation and then there are other special offerings at an additional cost. Sometimes it's a special dinner, a local show or a special behind-the-scenes glimpse into an historical site. Windsor Castle is one of the optional excursions available. Another popular optional in dinner in London followed by an evening cruise on the River Thames. Dan and I took that optional on our trip a few years ago and I think he would agree it was one of the highlights. There will probably be 5-6 optional excursions offered - a few in London and a few in Edinburgh. It's nice because you get to pick and choose and personalize our vacation. We don't all have to do the same optionals either. We can hand-pick the excursions that appeal to each of us. 
