What I can tell you about northern Italy (anywhere between Milano and Venice) is that the cities basically closes down in August (except for tourist destinations such as museums and churches), and even more so in the afternoon. If you will be walking the streets of any town in that area (Modena, Padua, Ravenna, Verona, etc), it will be like in a Zombie movie - except no Zombies. The city will wake up again in the late afternoon once the sunlight begins to fade. If you have been walking around in the afternoon, you will be ready to take a nap right when everyone wakes up and goes down to the street to take a drink.
So the best is to do as the locals do: Get up at 8:00-9:00 to get breakfast, buy something at the local markets, maybe have an early lunch, visit a museum. Then go to bed, take a nap, wake up at 6:00pm and start at the local cafe with a cocktail, stay up until 2am carousing the streets, stopping everywhere to enjoy, rinse & repeat.
Now Venice is a different animal. Since it is THAT popular with tourists, you need to prepare mentally and physically for what you are up to. ANY tourist destination at all, wherever you go, as opposed to even the most famous places west of Venice will be packed. And with packed I mean 2+ hours of waiting time to get in. It's literally insane. You will not want to do it. On top of that, the prices are ridiculous. Venice is best in October or similar, in August, all the world will be there, and you will be at the end of the line.
My recommendation would be for an unforgettable holiday, if you chose to go to this area:
- Get a list of the world heritage sites in that area, there a MANY.
- Book small hotels right between the towns, not in the town center, they hardly cost anything. If you want to spend more, there are AMAZING boutique hotels right downtown
- rent a cheap, small car, the distances are small anyhow
You will not have seen Venice, but you will have seen a large, culturally and historically VERY important part of Europe: The Opera of Verona, the 2000 year old mosaics of Ravenna, the city of Modena (with the Ferrari HQ), amazingly beautiful padua, ferrara and many more. You can spend a day in each city and never stop being fascinated about the architecture, the churches and the general feel of italian summer - with VERY few people around. They are ALL in venice.