Until bullbrand replies, I'll tell you about how my boyfriend and I almost got lost in Venice. We went this summer, stayed from morning till evening and head back home. It was worth it though!
Since I was also curious to explore Venice on foot, as much as possible, I looked it up online and saw that, to get to the main piazza (the San Marco Piazza) from the train station, you'd have to go across the first bridge you see on your right (you're standing in front of the train station, there's the water bus in front of you and on your left, there is a bridge). And from there, you will see street indicators taking you to Piazza San Marco.
Yeah, at least that's what the 'net says, we got lost like 100 times! But it was good, that's the whole point of exploring Venice
There are some indicators, yes, but we were more preoccupied with staring at the buildings and people and the gondolas (you will cross sooo many bridges to get to the main square!), we kind of got lost. But most streets are a dead end anyway, so you'd have to turn back anyway
We stopped at a cafe bar on the way too, they make good coffee in Venice
Once you get to San Marco, make sure to go up the tower, the St Mark's Campanile, we paid 8 euros to get in, you will see the city from above, it's breathtaking! We didn't see any other attractions from inside though, there was too little time...
However, we then went to take a walk along the Grand Canal, away from the crowds and from the tourists.... we took a left from the Piazza, took some pictures, walked some more and then we ended up walking in a park, where we found a secluded coffee house and had the best cappuccino ever! It's right near the Largo Marinai D'Italia, here's a park there, with some odd but interesting statues.
Then we ended up on the streets of Venice, again and somehow found our way back to the train station. it was a real adventure, we didn't have a map or anything, that's what made it interesting.