with the 72 hour visa am I able to leave Beijing to go to Great Wall? Or would this not be allowed?

HI, I am traveling from Milan (but I am Canadian) to Beijing. I will be staying for 72 hours and 10 minutes. 1:30pm on a Thursday to 1:40 on a Sunday. I believe I still meet the criteria as I won't pass the 00:00 mark on the Sunday, but is this risky? I will then be flying in to Beijing 2 months later to connect to another flight. I have a 3 hour connection time.

Must you register with the police after 24 hours? How do you go about this?
Thanks.