must sees while in Venice and surrounding area

http://italy4travellers.com/beautiful-bookshop-venice/

https://europeforvisitors.com/venice/articles/vi-sights.htm

Piazza San Marco in the evening.

Mercato di Rialto (Rialto Market) in the morning.

I've forgotten the age range of your group and whether there are any mobility impairments. There are steps up to and down from the hump-backed bridges in Venice, but walking is the best way to get around, one hopes seeing some back streets as you go. On such a short visit and with so many people (I believe others have pointed out that splitting up according to interests would be smart), I'd give a lot of consideration to what places are convenient to see beyond the Rialto Market and San Marco (where I'd want to see the interior of the basilica, including the museum, as well as the Piazza).

Unfortunately, the area between and around those sights is massively overcrowded during most of the day. You need to branch out from there to see the real Venice. People who make quick trips to the city and see nothing but that area, plus the path to and from the train station, all too often return home hating Venice.

Unless you board a vaporetto at its starting point, there is a substantial risk that you will not all be able to get on the same boat. There is a person on each boat who hops off at each stop and controls the number of people allowed to board. The boats are often packed, but they won't let them get dangerously overcrowded. Even walking down the over-packed narrower streets around Rialto and San Marco, it's going to be very, very difficult to keep your group together.

I would suggest taking a look at the itinerary for the RS Venice/Florence/Rome tour. That will give you a pretty good idea of what a group can see in two days., and it’s all walkable except for the trip out to Burano.

Aside from the usual suggestions, which are great, I recommend going to I Tre Mercanti to watch them make tiramisu. They have several varieties plus other gourmet food items that make great souvenirs. You can watch them make tiramisu from outside if you don't want to go in.