St Marks Square and the Basilica are absolutely beautiful and worth seeing. The highlight of our trip was the trip to Murano, where they hand-make glass. Find the man in St Marks Square that provides free tickets to Murano - the ticket includes a water taxi ride there, entry, and a small tour. The catch is that they want you to buy something, but the pieces are beautiful (even if you aren't into glass) and range in price from budget to madness, so it is worth it.
Burano is also highly recommended; it is completely off-the-beaten-track, and the little village is beautiful - houses painted in all the colours of the rainbow. Burano is where they make lace, and you will find many lace shops here.
Quick Tips:
Harrys Bar is listed in many places as 'a bar you have to go to'. Having gone there and paid 32 euros for two very small glasses of Bollini and being surrounded by many (not off-the-beaten-track) tourists and no Italians, we would recommend skipping it. If you really want to taste a Bollini cocktail, there are many other places that sell it, or you can even buy a bottle of it for a third of the price!
Best Way To Get Around:
Walking! There is no motorised transport on the main island of Venice, so walking is the best bet. You can also take a gondola ride (expensive but worth it!) through the canals, which is beautiful.
Other options are to can take a boat (water taxi or ferry) to another island for a look as well.