Why not give me a real world use case, with a code base to work off of? Put up a codebase on github, provide a set of user stories and requirements to satisfy (ie, "add a real time search using this data set and code base which works cross-browser and degrades in IE"), and ask me to add it, thus basically mirroring a normal development process.
As someone who has had to interview programmers before, that would tell me infinitely more than their ability to re-iterate the word "Hoppity" on the command line.