I have been to Venice three times and each time it seemed busier. However I disagree you can see it a day trip. You can see the main tourist bits but to really see Venice you need to get away from the hotspots and explore the back streets especially at night when it empties out And is gloriously atmospheric and peaceful. Also the islands are beautiful and you could easily spend a whole day on my favourite Burrano which is so beautiful. My only issue with Venice is we struggled to find decent food - except for breakfast and lunch where we ate at the little places the locals go to. The last time we left, as we headed up the grand canal I felt really emotional as we will probably never go back - Venice is just unique there is nowhere quite like it