Loch Ness and it's famous Nessie is one of those mystery places you have been hearing about since childhood and it definitely can't be skipped when visiting Scotland. Since you won't be able to see any real monster (ok, except of some weird monster-looking tourists), you should think about your schedule and what you want to do in advance. I recommend adventurous horse riding (Scottish horses are really crazy and almost got me killed) around Urquhart Castle, getting drunk - eh, I mean tasting local whisky and having lots of funny conversation with the locals (when you won't be able to understand their accent - especially when you are drunk from that delicious whisky :-)
Edinburgh - the capital of Scotland is a must see on your trip in United Kingdom. The perfect itinerary in Edinburgh offers mostly history and culture. I love this city for it's royal feel, friendly people and ocean breeze.
Scotland differs from the rest of UK mostly by it's hilly surface. This is the place were golf was invented, where Dolly the sheep was cloned and where penicillin was discovered.