Yeah I've considered doing this a bunch of times but my biggest problem is coming up with small chunks of work that require little-to-no domain knowledge and can be done in a day/weekend/week.

We've switched to having candidates do two take home problems that are derived from actual problems we faced and it's worked very well. It cuts down on the number of bad candidates that get to the office and waste an hour of time and it gives a chance to candidates who are good coders but bad whiteboarders.