When you're in Venice, be sure to take a half-day trip to see the glass-making factories on the nearby island of Murano. A waterbus from Venice to Murano will take you about 20 minutes. There are many factories so just follow the signs. The glass-blowing demonstrations take less than 10 minutes and are absolutely fascinating. After your glass-blowing demonstration, you will be shown into the glass store to buy there products, but know that there are at least 50 different glass stores on the island of Murano, so feel free to window shop!
Also, as a side note, you can get most of the same stuff on the island of Venice at a cheaper price -- you just have to make sure its says "Murano glass" if you want the real thing.