Flight: Germany>PEK>HKG / Back: HKG>PEK>Germany

I have booked 2 connecting flights:
Towards Hong Kong : Germany > PEK (Transit 12hours) > Hong Kong
Flight Back (5 days later): Hong Kong > PEK (Transit 3hours) > Germany.

On my flight towards I want to have a quick city-view of Peking/Beijing.
I have a Belgian passport. Do I need a VISA or does the 72h free transit (for each way!) do?

Thank You.