The free excursions are to Murano and Burano, and as Doug stated are run by the commercial glass factories. Very popular with the cruise ship folks. You get a short visit to Burano and then a long stop on Murano to visit a glass blowing place. You will have to take the tour and then spend a long time in the gift shop before the boat takes you back to Venice.
Burano is 40 minutes by boat. It's a beautiful island of painted houses and known for lacemaking. My suggestion is to follow the other suggestions, get an early boat, walk around a bit, have lunch, and return early afternoon to continue exploring Venice.
Skip Murano. If you really want to see how glass is blown there are a few places in the heart of Venice that would love for you to take their tour. (No boat necessary.)