" . . . just go, stay few hours and come back, and i dont know if i may do police registration, and if i do, where may i register as the place i will stay."
It doesn't sound like you're staying anywhere: not in a hotel not in a private place. Homeless in Shanghai as it were. The police registration needs an address, needs an owner of some sort at that address. Just try to register in your situation and I have no doubt you'll flummox the people at the paichusuo.