We do something similar around each major release, we call it Branching Day as it's when we create the maintenance branch in cvs. We branch on the thursday and each developer is free to try something new out to kick off development on the next version until friday afternoon. If there's much worth showing we get the company together for some beers and demonstrate what we've come up with. It's normally backendy or code tidying kind of stuff which might not otherwise might not get pushed up the priorities or some visual nicety we know people will like but again are unlikely to ever shout for.
