In Venice from 8 am to 4 pm

Yes, with so little time, just walking around is the best thing you can do.
It it was me, I would ignore the hotel breakfast, get up early, like 6:00 and head to the Rialto market on foot. Easy to find, follow the signs saying “per Rialto”.
Have a coffee and croissant near the fish market.
From Ralto, walk to Ferrovia, and take a No 1 vaporetto the length of the Grand Canal to the Arsenale vap stop.
That is about three of your eight hours used up.

Also, check the cruise ship schedule of the day you will be there. If there are a lot of ships in port, avoid the st. Marks and Rialto areas between 10 and 5, as they will be wall to wall people. But, walk away from those areas ad things quiet down and are amazing. Dorosodoro, San Polo or Cannaregio are much more typical and don’t get so mobbed with day trippers.

Andy

Ditto on the up early stay out late comment. I am in Rome on the last day of a 14 day solo trip to Italy planned using all of Rick's tools, with three nights in Venice. I went to the Doges Palace and St. Mark's Basilica late in two different afternoons at 3:30 and 4:00 PM respectively and at St. Mark's found a 10 minute line to get in (no backpacks) and virtually no line at the Doges after a boat ride to Murano. You can also try both of these places, which are well worth your time, very early in the morning when they open. Cruise crowds do make a huge difference.

The Rialto was no big deal for me, just a lot of crowds and you can get a better picture of it from the water. Next to the Rialto, however, is the Fondaco dei Tedeschi, a multi-level upscale shopping mall and on the fourth floor they have a terrace overlooking the Rialto and the Grand Canal on both sides of it. Incredible views of the GC and all of Venice. It is free, but you need a reservation to get on the terrace. You can make one by going to

https://www.dfs.com/en/venice/stores/t-fondaco-dei-tedeschi-by-dfs?cid=sem-pre-goo-vce-en-l-rome-sg-dynamic&mckv=scvc2Y1mq_pcrid_191774224761_c_slid_&gclid=EAIaIQobChMI0oSdn7OW2wIVER0YCh2wnA9fEAAYASAAEgJ-hfD_BwE&gclsrc=aw.ds

I would say no to the guided tours because you'll be in a slow-moving group following around someone with a microphone and an umbrella in the air most of the time. With your limited time you need flexibility. Rick's audio tours will be better because you can speed it up past something that is not of interest to you. He gives a lot of historical information, but with such a short time you should really BE there and not worry so much about the history you can learn about later. Are your times set in stone or do you have flexibility. The Church of the Friari recommended by Rick was a welcome surprise and its a short walk from the Rialto on the opposite side of the Grand Canal. As good as St. Mark's but in a different way. Check out Rick's doing Venice in one day tips on this web site. Just type that into the Forum search bar and you'll find it. By the way, you called this a snapshot tour. Be sure you don't get so caught up in taking pictures that you miss seeing what is right in front of you. Unless you're taking the pictures to show someone you've been there the memories (sights, sounds, smells) of what your eyes see and store in your brain will last a lifetime. Hope this is helpful. Have a great trip.
Jim

Thanks for your replies, really appreciated! Just to clarify, "snapshot tour" is not in the literal sense, but I meant to say just a quick tour.

We would really like to finish St. Mark's square and the surrounding area before the crowds get there, since our hotel is close by. I'm not sure whether it's worth seeing Doge's Palace from the inside, unless one has a lot of time to see the art. Many of the big name tours seem to see that area from the outside, including the Bridge of Sighs.

Here's what I have on our list so far, a work in progress:

1. St Mark's (early in the day).
2. Doge's Palace
3. Walk to the Rialto and the Fondaco dei Tedeschi, as suggested by Jim.
4. Walk or take the vaporetto to Ferovia and return to San Zaccaria by 4 pm.

Is there a good spot to have lunch, overlooking the Grand Canal?

When do you arrive in Venice, when do you depart?

We arrive in Venice at our B & B (near St. Mark's) around 8 pm on day one. We can walk around a bit, but besides getting some food, don't want to plan anything that night. Being that it's "vacation", I don't think our traveling companions will be ready to arrive at St. Mark's before 8:30 am the next day. We need to be back at the B & B to give us enough time to pick up our bags and head to the train station to catch a 5:25 pm train to Florence that day.

And how do you arrive and depart, (train, car, bus, airplane)?

There is an area on the Grand Canal called Fondamenta Vin Castello. It’s close to the Rialto and has several bars/restaurants with outside tables. The views are great. Al Pescador is one of them but I wasn’t impressed with the cicchetti. I’m giving you the name so you might be able to look up others on the same fondamenta. If you go to the terrace at the department store - which you should. Go back over the Rialto on the side closest to the department store and follow the canal until you see all the tables outside.

There are also several restaurants on the canal on the other side of Rialto but I would steer clear of them. And, of course, the very expensive hotels closer to St. Marks have outside restaurants on the canal but I am not thinking I would want to pay their prices.
There are not very many restaurants on the Canal, unfortunately. The best restaurants are pretty hidden away.

Wear comfortable shoes. Have water. We spent most of our day in Venice lost, just wandering the maze that is the city.

Sam, we arrive by plane and depart by train as I mentioned earlier, and plan to take the boat from the airport to San Zaccaria, where our host will meet us.
One place I forgot to mention that we will go first is the Campanile Tower, before St. Mark's.

Starting at 8:30, you'll spend a lot of time waiting in lines. I suspect that you and your traveling companions will leave with a very unfavorable impression of Venice. If you are going to the train station to depart, why take a vaporetto to Ferrovia, then back to San Zaccaria, then back to Ferrovia to get your train.

At least get up early and take a walk to San Marco at 6 am so you can see it without hoards of people, go back and have breakfast, then return and be amazed at the crowds

If you are planning on the Alilaguna to San Zaccaria, that is also not an optimal. 1 hour and 20 minutes in a crowded boat with salt caked windows making several stops. How many traveling companions? if 4 or 5, think about splurging on a water taxi for about 120 EUR. Otherwise, the bus and No 1 vaporetto is my normal way. There are faster vaporettos around through the Giudecca Canal, but taking the "slow" boat down the grand Canal is pretty special, first time there.

Appreciate the advice , Sam. I'll look into the bus and vaporetto as an alternative from the airport. If my co-travelers don't want to get going early in the morning, and knowing our time limitations, what sequence would you suggest for St. Mark's, Campanile, Doge's Palace, Rialto, and the vaporetto ride? We plan to buy skip the line tickets for St. Mark's and Campanile, but will just do the basic tour and skip the museum. The earliest skip the line tickets I can get are 9:45 for St. Mark's and 10:30 for Campanile. Just wondering if we should just line up at the Campanile well before 9 (since it opens at 9), take that tour and book St. Mark's for 10:30? Also how long should we allow at each place?