Because the purpose of an interview should be to best simulate a situation that will give evaluators the most accurate view of how a candidate really behaves, it seems to me that getting them out of the office and doing something slightly more natural and unconventional would be a better idea. Heck, even taking a walk or going shopping is better than sitting behind a desk. The key is to do something that provides evaluators with a real sense of whether the person is going to thrive in the culture of the organization and whether other people are going to enjoy working with him or her.