The Muji Chronotebook combines the flexibility of a plain paper notebook with the utility of a daily planner.
For each function or feature you add, you lose a purpose. A blank sheet that could've been used in a million different ways can now only be used for a few. Artists aren't going to buy a calendar if they're looking for something to sketch on. Writers aren't going to pick up to-do lists to use as a journal. This isn't a bad thing per se -- by narrowing down on a purpose, a blank sheet of paper can become more useful and relevant to certain people.
Each page of the Chronotebook has a analog clock in the middle, around which you can freely form appointments (just draw a line to the time for the meeting), sketch, make lists, or anything else the mostly blank page beckons you to do. Fantastic idea.
Update: Here's the same idea in whiteboard form. (thx, michael)