Does anyone know where the best places to eat lunch in St. Marks Square in Venice?
Personally I wouldn't actually. All the places at St. Marks are extremely expensive/over-priced. I would instead recommend that you go a block or two away off some side alley and find a place that is perhaps a little less touristy.
San Polo is a great choice to find some neat places. We enjoyed Trattoria Alla Madonna in two past visits.
I agree - most of the restaurants on or near St. Marks have insane cover charges PLUS most charge a 12% service fee on top of already expensive food. Find something a little off the beaten path. You'll still be close to the action.
If you want to eat in St. Mark's Square, save it for an after dinner drink when you can sit for a long time and listen to the music. You'll pay a cover fee and the drinks/desserts are pricey, but you're paying for the ambience.