I think a lot of this depends on how you sold the scope of the project to them before. If you are truly on an hourly basis all I would do is give them a "general overview" of what you were working on accomplishing for the day and leave it at that, anything more and you might get too technical, or could back yourself into a corner.
For example, below would be something that I might put out to a customer.
9/1/2010 - Initial project setup, architecture, and source control configuration - 4 hours
9/2/2010 - User Interface definition and prototype creation - 5 hours
9/3/2010 - Database work - 4 hours
I personally try to not get too far into the weeds as I find it confuses more than anything. Then they can ask you for more detail if they really want it.
However, I should note that in general, I simply would do a weekly invoice of "Worked 34 hours on __ project" unless they wanted the extra detail. (Now I have a bit more freedom typically with the way I work, but they don't typically need to know the start/end times and other things.