Edinburgh is a must see if you're in Scotland, one of the most beautiful cities in the world (in my opinion) with loads to see - The Castle, Holyrood Palace, Scottish Parliament (only if you want to see one of the strangest buildings around), Mary Kings Close, Greyfriars Kirk, The Royal Mile, Botanic Gardens, loads of art galleries and museums. And lots more.
Aside from that, the west coast of scotland is pretty spectacular - Oban is a great little place for a one-nighter and you can catch ferries from there over to the Isle of Mull and Iona.
If there's a whisky lover in the group, you could take a drive through the speyside valley. The scenery is pretty amazing and every now and then you drive past a distillery - most are open to the public for a tour and a wee dram. (It would involve a drive from somewhere like Aberdeen to Aviemore - A95)
Hope that helps
caroline