You should see Venice if you never have before. In fact, read up on its history first, it changes the experience. Everything from how they built it (those trees they drove into the ground didn't grow on the marsh!) to the rise and decline of the empire. Knowing a little about what you're standing on just makes a cool city a really different experience.

October is still cruise ship season in Venice, IIRC. Perche would know better. That means you need to get out early before the ships arrive, and then again at night after the day trippers have left. Get lost for a while and see what you can find to eat and drink.

I take a slower pace than most. If I had a week, I'd probably stay in one city. In your case, maybe a night in Milan then off to Venice for the duration. I try for maybe one tourist sight per day, but then wandering the rest of the day. It's one thing to see, but another to experience why it matters that you're there and not just looking at pictures.