Get a map when you arrive - everything is within walking distance. You will see most of the major sights in 2 days no problem. Also, a 2 day pass for vaporetto (water bus) will save you a few bob. You can buy this at Piazza le Roma. Be aware that around San Marco, meals etc are a good bit more expensive than in other areas - make sure you check out prices in the window before you go in and sit down.