1) Gondola for €0.50
There are private gondola ferry services that cross the Grand Canal at six different points. (These are often marked on the maps by a dotted line). Just turn up at the embarcation point with your 50 cent coin. The crossings take no more than a few minutes and you usually have to stand up. Brilliant fun!
2) Never sit down for a coffee
In the centre of Venice never sit down at a café or bar for a coffee - ever! If you do you will be charged a hefty service charge. Do as the Italians do and have your drink standing up at the bar: You will be charged no more than the price set by the "Friuli Venezia Giulia" Region - currently around €1.00 for an espresso and around €1.20 for a cappuccino - (Starbucks schmarbucks)!
If you really have to sit down check the prices which must be displayed by law in the bar. There will be two prices quoted: 'Al banco' (standing at the bar); and another one usually referred to as 'Servizio a tavola' (where you will be served when sitting down). Some bars in Venice have modest and reasonable table service rates - but some are extortionate! (Don't say you weren't warned!)
3) Prebook Vaporetto tickets
The Vaporetto is expensive, but if you have to use it try to prebook on line.
http://www.veniceconnected.com/node/1503