If you visit Venice you’ll probably end up in Piazza San Marco (St Mark’s Square) which is the main square.
This is where the impressive Patriarchal Cathedral Basilica lives. You can wander through the church which has an incredibly ornate decor inside, but you can’t take pictures inside, but you might have to queue. Even in October there was a constant influx of tour groups getting off boats and flooding the square and I imagine in the summer it is very busy.
You’ll also fine the Campanile which is a tower nearly a km high and apparently features in Assassin’s Creed II, below are the views from the top. It cost about 10 euros to go up and there are lots of stairs! And all you get is the view so make sure it’s a clear day!