Registration Public Security Bureau

Hello,
I'll have a stopover in Beijing for about 12 h and would like to visit the city. I am eligible for the 72 h/144 h visa, but I have a question: Usually a registration at a police station is required. If I would stay in a hotel, they would register me automatically. But as I don't stay overnight I didn't book a hotel.
Does anyone know if I have to register at the local police station? If so, where exactly can I find the Public Security Bureau in Beijing?