The best way to get around Venice, apart from strolling aimlessly along the lanes, is to use the ferry system. Tickets are sold at major ferry stops like Piazzale Roma, Ferrovia and Rialto and include a time limit. Current prices are:
1-hour €6,50 one-way only, excludes bus
1 1/2-hour €10 one-way only, includes airport bus
The longer fares allow you to use any of the Actv ferries or Lido buses and is also a convenient way to see some of the city from the water. There are also airport aerobus tickets available, generally for a few euros more.
Tickets are validated each trip at a machine as you walk onto the wharf. Mind you, knowing which wharf to board a ferry can be confusing at first as some of the bigger wharves use different ones depending on which direction the ferry is going.
And keep in mind there is a luggage limit of one large suitcase per person with an extra €6,50 charge per bag.