The best way to get to Venice from Florence is to take the train.   Trenitalia (www.trenitalia.com) is the Italian national train service and they run trains about every hour there from the Venezia Santa Maria Novella station into Venezia San Lucia station about every hour or so during the day.  Buy your tickets on the day of travel and it will be 45 euros.  Buy them in advance for as little as 19 euros each way.  The trip is a little over 2 hours.  As an alternate, try the new upstart train company, Italo (www.italotreno.it) runs about 5-6 trains to Venice daily at competitive prices.

Once arriving in Venice, step out of the train station right into history.  I always find it surreal to be in Venice - there are no cars, heavy boat traffic on the winding canals, people and markets everywhere.   I recommend making an immediate left out of the train station and following the well-marked signs to Piazza San Marco.  By doing so, you will walk through some of the most interesting places in the historic district of Venice.   It will be hard to miss the Rialto bridge on your way to San Marco. 

How long will it take to walk from the train station to San Marco?  Who knows?  I suggest following where you feet want to lead you, and don't forget to stop for a nice, long lunch but get off the tourist path to find something more authentic and somewhat cheaper.

Once arriving at Piazza San Marco, take it all in.   It's free to visit the Basilica.  Other sites not to miss are the Doge's Palace and the romantic Bridge of Sighs.  It's expensive, but a cafe in the Piazza is a memorable experience.

When you are ready, hop on one of the vaporetto for an hour inexpensive cruise down the Grand Canal back to the train station.  It's a worthwhile trip to Venice that will surely be enjoyed on your next tour to Italy.