At the risk of sounding like a Phillistine, we recently did a half-day trip to Venice (at the recommendation of our local holiday rep) and found that to be as much as most people in our group wanted. We had 4 adults, 4 teenagers and 1 under 10. We arrived there (as suggested to us) at around 4.00 pm when most tourists were finishing up for the day and leaving, so it became much less busy, took a long water taxi ride right down the Grand Canal, had picnic in the Gardinieri (sp?) then strolled back up from there to the train station, taking in St Mark's Square, a meal, drinks, the Rialto bridge, etc, and took the 9.45 pm train out of Venice. I'm sure we missed lots, but we also saw and experienced lots, and really enjoyed the visit there, but we had other things to do too and time was short. No regrets!