There's a joke I remember from Letterman years ago, from the Top Ten Consequences of Growing Old. One of them was "Sooner or later it's your turn to marry Zsa-Zsa." At my house it's constantly modified to suit the occasion, as in "Sooner or later it's your turn to get a colonoscopy," or "Sooner or later it's your turn to visit Venice." Which reminds me that thematically, Venice and Zsa-Zsa have a lot in common.
I've been to Venice enough times since the '80s to be comfortable getting around, finding restrooms, eating relatively inexpensively, etc. Someone who cruises asked recently at a forum about how to see Venice when the cruise ship will be there for a single day, 7 a.m. to say, midnight. As we had just the week before departed Voyager of the Seas early in the morning to spend a long day in Venice, courtesy of My Wife the Travel Agent, I was able to give a timely answer which may be useful when it's your turn to marry Zsa-Zsa.
This itinerary starts on a cruiseship, but if you aren't on a cruise, that's ok: just ignore the parts about getting from the ship to Piazzale Roma. If you come from Marco Polo airport via taxi or bus, you'll be dropped off in Ple. Roma anyway. If you arrive by train, I'll cover that too.
Starting at a cruiseship in the morning:
Eat breakfast on the ship. Bring a hat (or umbrella if the sun really bugs you) and a map you've already gotten familiar with. The places I refer to can all be found on any map of Venice.
Walk out of the cruiseship terminal (there may be a free shuttle). Once you are out of the cruise terminal, head for the PeopleMover station, a new elevated monorail. Buy two 1€ tickets (one for the return trip) from a ticket machine (cash) and take the PeopleMover to Piazzale Roma (public restrooms at P. Roma near where the image below says "Your Taxi stands here").
At Piazzale Roma, walk left out of the PeopleMover exit for a couple hundred yards to the ticket booths for the the vaporetti (water buses). You have to go past assorted bus ticket windows and down some gentle stairs to get to the water where the vaporetti booths are.
Buy a 24 (12 hr pass might not last long enough) hour pass (they take creditcards; let your card companies know when & where you'll be traveling. I don't like to have any cash out in Ple. Roma.) Walk a few more steps to the vaporetto stops, find the red-circle-2 symbol on one of the floating waiting rooms, activate your pass cards (watch other people do this) and get on vaporetto #2 to Piazza San Marco. #2 runs in a circle both clockwise & counterclockwise; you take the clockwise route to start. See map:
You might copy this #2 route-map and take it to Venice. The regular vaporetti route map shows all the lines and is less easy to read...like so:
For this day in Venice you will take only the #2 vaporetto. It saves a lot of heartburn, hits the hotspots, and you won't have time to see everything along the #2 route anyway, much less the other lines.
We like to sit on the open-air seats at the back of the vaporetti; you might give it a try.
The whole process of getting from the ship to actually sitting on the #2 vaporetto can be stressful; just relax and know you have plenty of time.
Remember, you can just take the #2 all day and not worry about the other lines. Per the #2 map you'll get on at P. Roma. If you came on the train, it's a bit simpler. You'll walk straight out of the station and (almost) fall into the Grand Canal. Buy your pass at the ticket booth on the water, and get on the #2. This stop is called Ferrovia (iron-way, railway). From the #2 map you see it's the stop right after Ple. Roma, going clockwise. The clockwise route will take you down the Grand Canal to Piazza San Marco, where you'll get off.
Piazza San Marco by Renoir
I like this shot of San Marco; at the upper right you can see the original brick church behind the fanciful facade which was added centuries later:
At this hour of the morning it shouldn't be too hot or crowded. Also the lines to get into the church and the Doge's Palace should be short. You can also aimlessly check out the streets around the church & square, as well as the pleasant views along the Canal. It's hard to be lost for long.
When you've had your fun, walk from P. San Marco to Rialto bridge. It's a wander, there are a few ways to get there, and don't worry about getting lost, there are directional signs painted on the buildings. There are also shops & stuff on the way, and public restrooms near San Marco. Also many ATMs; I use my regular bank card to make withdrawals, and the machines have an English option. Generally stores take creditcards; street vendors want cash; public restrooms want 1.5€; restaurants are ok with creditcards, but the waiters want cash tips.
On the Grand Canal by the Rialto bridge on the St Mark's side (before you cross over) there's an affordable self service cafeteria if you want lunch or something to drink. It has a restroom for patrons. In this photo the cafeteria is to the right of the Hotel Rialto:
There are shops, etc., on both sides of the bridge, and on the bridge itself. Cross over the Rialto bridge. At this point because I like art & churches I'd walk over to the Basilica di Santa Maria dei Frari. From there I'd go see the paintings around the corner at the Scuola Grande di San Rocco. So much of the art is on the ceilings that you can borrow a mirror so you won't wear out your neck.
After all that edification it's time for sitting down & having refreshments at Campo Santa Margherita. Once restored, head to Accademia Bridge (public restroom). From Accademia you can:
1. Walk over to Santa Maria della Salute church (the big white-domed one):
Salute by Canaletto
I like Salute (sa-LU-teh). By the way, the name means Saint Mary of Health. The church was built after Mary's intercession spared the city from the worst ravages of plague. You may notice on the #2 map that there's a Salute stop, but it's not on the #2 circuit and so I don't count it.
2. Or cross over Accademia bridge and take a gondola ride right on the other side (I think it's worth the money to silently glide in those narrow side canals):
...and the bridge- it sighsSigh.
Also at Zaccaria is San Zaccaria church, which has the relics of John the Baptist's father.
4. Or take #2 out to the Lido stop, and promenade along the lovely Gran Viale Santa Maria Elisabetta with a gelato. Walking to the beach and back is pleasant and not too long. The Lido is not like Venice, and has its own charm, having been a glamorous resort going on 150 years.
Many of the houses & hotels have a light Viennese character in contrast to the more sober textures of Venice proper. Returning from Lido to Venice on the vaporetto also provides a seaward, historical approach to the former Mediterranean power:
La Serenissima from her best angle
San Giorgio by Canaletto
Il Redentore (the Redeemer) by Canaletto
For dinner I suggest taking the #2 (or walking) to the Zattere (or Palanca) stop, and dining at one of the outdoor restaurants there. Zattere means "rafts:" it's the wharf where rafts once delivered bulk materials to the city. Now the only rafts are those which provide restaurant dining right on the water. The breeze is welcome, the view across the Giudecca Canal (you can see both of the above-mentioned churches) is gracious, and the prices for pizza are not paralyzing (but check the posted menus). A bonus is that the cruise ships all tiptoe through the Giudecca Canal to reach the Adriatic, and you may see this happen. Some ships are so crazy big that they ruin the scale of the city:
Wave at the people on the ship
Also at Zattere is La Chiesa dei Gesuati, well worth a peek and a prayer if it's open. If you do dine at Zattere you might then take the #2 across the Giudecca to the Palanca stop, and stroll over to the Hilton Molino Stucky for a pricey drink and included hors d'oeuvres at the rooftop bar with its splendid views (again, I thought it was worth it).
It was once Conrad Hilton's turn to marry Zsa-Zsa
Once your day is done, if it's not too late take the #2 back through the Grand Canal again before getting off at Ple. Roma: at night it's more mysterious...eerie...morbid, even. Plus you can see inside a lot of the grand main rooms of the palazzi. Let's say you had drinks at the Hilton. You'd be near the Palanca stop. From Palanca if you had time, you'd take #2 counterclockwise to see the Grand Canal before getting off at Ple. Roma. But if you just wanted to get on the ship, you'd take the #2 clockwise back to Piazzale Roma (and then the PeopleMover back to the cruise terminal.) If you're taking the train, you'd get off at Ferrovia.
Here's the easy way to tell if a vaporetto's route is clockwise or counterclockwise: let's say you're at Palanca (per the #2 map). If the boat is coming from the left, from Zattere, it's counterclockwise. If approaches from Redentore, to the right of you, it's clockwise.
The regular vaporetto routes cut way down at midnight or so; get on before then.
My strategy for the day is to stick with the #2; strike a balance between walking and vaporetting; and between gotta-see and wandering. I've listed more than can be done in a day, so don't run yourself ragged. Also consider that sometimes you can walk straight between points such as Accademia & Zattere, or Rialto & San Marco more quickly than you can take a vaporetto.
Of course, your mileage may vary, and you have my blessing to do your own thing.
Also have a look around Venice using Google Earth; it's a great way to get a sense of the city before you visit.