I visited Venice for the first time last month. We spent three days there and it was the perfect amount of time. I definitely recommend paying for a guided tour of St. Mark's Basilica. You can enter on your own for free, but we paid for a guided tour and it was well worth it to have a knowledgeable guide explain the historical, architectural, and religious significance of various details. When we were in line to enter, a woman was rounding up a tour and we just joined in for about 10 euros per person for a 90 minute tour. Also in St. Marks, we toured the Ducal Palace and also rode to the top of the clock tower (amazing views of Venice, worth a ticket to the top). I suggest taking one day to just walk around and explore. We had two fabulous dinners in Venice - Osteria alla Botte and La Mascareta Osteria Enoteca. I highly recommend both, and while they are slightly set back from the main square, they are within reasonable walking distance.