Feats of strength! Interview is not over until they pin you. :p
Ok, the best thing I have both implemented and done in interviews is code. Do two things: 1) give them some code from an example app and have them reason through it for you. Don't be afraid to probe or ask clarifying questions. 2) Have them talk through how they would build an example application. Focus on the big picture as well as some nitty details like what certain logic or markup might look like. Have them talk through this with a piece of paper or white board. Throw in some curve balls and see how they react to the changes.
There are other things you could do but honestly I've found this to be the best use of your and their time. You'll get an inside look at their creativity and logical reasoning and they'll be given a chance to show what they know and "geek out".
Last, have fun!!