I'd suggest climbing Arthur's Seat...if they're up for a little exercise. Might as well tour Holyrood while they are there, too. I think there is a special exhibition at the National Galleries of Scotland...a collection of images and art of HM the Queen if they are interested in that. Will they be there during the Fringe Festival? There might be some shows there that they'd like to see. The Museum of Childhood on the royal mile might also be worth a visit. Last but not least, they might want to make a reservation at The Witchery for dinner. Might be a little expensive for what it is, but the atmosphere is awesome (and don't forget to get the passionfruit trifle for dessert!)