Venice is a small area and it is possible to walk from one end to another in a couple of hours. It's worth doing in order to see the quieter views of the city. Whilst San Marco is indeed a golden and beautiful sight, the tourist orientated stalls and gondolas do give it a gaudy feel. The Carneggio district is a world away from this with peaceful canals and bridges occasionally interupted by working boats and people. I would recommend a walk around the lesser known parts of Venice, paticuarly in high season when the tourists do become a stifling mass. Venice is a beautiful place and a lot more is to be gained from an out of season visit than you might expect.
Quick Tips:
Book ahead in summer although the Convent hostel on Guidecca usually has some vacancies, along with some very eccentric but lovely nuns! The further away from San Marco the cheaper the prices, for everything!
A trip to Burano is bountiful and a lot quieter than Murano.Best Way To Get Around:
The water buses are cheap and regular and its pretty easy to figure out the system. However DO NOT get caught without a ticket or you will pay a hefty fine, patience with tourists is low on this point.