One thing to bear in mind is that Venice can be expensive, perhaps in part due to its popularity, so it's a good idea to look for package holiday deals. That way, the saving you make on your package holiday in Venice might negate any extra money that may be spent there. There can be a large mark up on the price of drinks at Harry's Bar near St Mark's Square, for example, but it could be worth it for the experience. It is the birthplace of the Bellini, after all.
In Piazza San Marco itself is Doge's Palace, where visitors can have a guided tour. A must-do activity in Venice is taking a ride on a gondola – it is one of the best ways to see the city and few would want to return from a holiday in Venice having not experienced it. There are shopping opportunities in the city as well, such as that at the Rialto market at San Polo. For cultural experiences, there is certainly no shortage of interest in Venice. Whether it is classical music, art or architecture that takes your fancy, you will find it here.
If it is nightlife you're after, Venice is not the place for you. There are bars and restaurants, but they can be tricky to find. You may have to stray off the beaten track for bars that are open late, and some cafes are open until the early hours.