We also got up early and started roaming the streets and walked over to St. Marks. Got there about 8. Only a handful of people and lots of pigeons. Quiet, special, surreal. It’s well worth it. We stopped at about 3 places along the way for a cafe! Day trippers arrive about 9. You will want to get in line to the campanile bell tower there before the crowded. 360 view of Venice, very special.
Also, go back at night after dinner. Sit by the orchestra at the Florian, have dessert and a drink and enjoy the ambiance. You’ll have to pay to sit, but it’s fabulous.