Venice is riddled with narrow alleyways and canals - There would be spots along the major canals and in front of the larger churches that might offer opportunities for wide panoramic shots. It really depends on what sort of thing you are looking for.
I spent a little over a week shooting 5x4 in Venice not too long ago - Never had a problem using a tripod in public areas. Photography inside churches and museums was out of the question... That said, i did find a good vantage point from the bell tower at the Church of San Giorgio Maggiore. No where near as popular as the St Mark's Campanile and I had no trouble using a tripod up there. Tip if you decide to go up the tower. Wait until just after the hour or you'll get deafened by the bell.