Im latin guy and end of the month will go to Poznan and ask her if want to married me, but I don`t know if you can suggest me an idea! where for example.
Stary Browar. Or failing that, Rondo Kaponiera.
Okay, I'll be serious. Poznan isn't really too great for such things - although it depends entirely on what you want. Nice restaurants are commonplace, but there's nowhere really completely romantic - all depends on the girl. But you probably can't go far wrong with some of the restaurants located around Plac Wolnośći, I think.
Just not Stary Browar, whatever you do.