With the pubs running a close second, of course!! I returned from Scotland, including Edinburgh, a few weeks ago and would see the castle, but if it's cold or raining, I would also recommend the Scottish National Portrait Gallery if you're into that kind of thing. We really enjoyed a few hours we spent there. There are a lot of places on the Royal Mile to duck into (museums, displays, cathedrals, etc.) if you need to warm up or dry off. We loved the stained glass at St. Giles. Then, take a train somewhere, anywhere, and just get out and walk around. There may be a cathedral or castle nearby and definitely warm, cozy pubs and friendly people. Have a blast!