Does it have to be work related? Also, do you end up showing off whatever you did during that time, or is it really a "do whatever" day where you could just do nothing?
At the end of the second day, there is usually a little show-off session where folks that got something interesting at the end and want to share, they can. But you are not forced to.
It’s still normal working hours, so you are expected to do something.
At the company I work for hack days can even be used for you to read technical literature, so if you'd like to take these 2 days to grok through a technical book you haven't found the time for you can do so. Is that encouraged where you work or does one need to "build" something out of these hack days?