How do I fill 2.5 days in Venice?

The highlight of Venice is just walking around or taking the boats around - it's such a unique place, with all of the little bridges and piazzas and churches, with no cars or bikes. You wouldn't need to go inside a single museum or church to enjoy it, in my view.

Two and a half days isn't much time. Sounds like you have your "must see" things already listed. Otherwise, I see no point in trying to over-schedule it. Relax and soak it all in.

Just keep in mind that the big tourist spots like San Marco get mobbed with tourists, and it can be unpleasant. Try to explore Venice in the mornings and evenings when most of the day trippers aren't in town.

Murano is not a long boat ride away from Venice so easy to see, so I'd probably do it (get a vaporetto day pass that day most likely). But it's not really that special in my opinion. Burano on the other hand is really lovely with its colorful buildings and worth a visit...if you have time. It's a much longer boat ride away by vaporetto, so getting there and back will eat a lot of your limited time in Venice. I probably wouldn't do it unless you have already seen everything else you want to see and are just tired of the crowds or something.

Are you buying vaporetto passes? Riding the Grand Canal at night is beautiful! You can ride an elevator to the top of San Giorgio Maggiore-great views of San Marco and beyond, plus the added bonus of fewer crowds. The Clock Tower tour is interesting, limited to groups of 12, and overlooks San Marco. I'm sure your family will enjoy gelato, whether they decide to try different flavors or compare the same flavor at different shops. Burano and Mazzorbo islands would likely be more interesting than Murano. Perhaps others will share about Lido, as I haven't been there. You are wise to not overschedule, as Venice is a great place to enjoy at leisure.

Andrew has it right, I think -- just wandering around, especially early and late in the day, and away from Rialto and San Marco, will bring many "discoveries." Vaporetto passes will make it easy to ride when you're tired of walking. Under "Explore Europe" on this website you'll find a list of Venice sights others have enjoyed, but your choices may vary. Going into any church you'll find good, sometimes great, art and architecture.

I haven't been to the Lido so I can't compare it to other beaches, but if I lived on a beach I wouldn't cross the ocean so see another one, especially so close to a unique city like Venice.

You and your children might enjoy a mask-painting class. There is also a working gondola repair shop in Squero San Trovaso which I think is quite unique.

You might consider the Secret Itinerary tour of Doges- it was really interesting and very fun-
https://palazzoducale.visitmuve.it/en/pianifica-la-tua-visita/special-itineraries/secret-itineraries-tour/

You can book a timed entry to St. Marks for 3 euros- well worth it.
https://basilicasanmarco.insidecom.it/en/attractions-in-veneto/tickets/skip-the-line-saint-marks-basilica/#choose

We booked entry for just after opening at St Marks- get to the piazza early before the crowds,enjoy it then see the Basilica without a wait. Be sure all knees and shoulders are covered- they are very strict- boys too!
And no backpacks of any size allowed- you can check for free across the plaza/down a lane.
Then we did the Secret Itinerary at 10:45 I think- after the tour you are free to see Doges on your own. Avoids any wait in line.

That takes you til about lunch time- still time to ride out to Burano for a few hours and return well before dinner.
If Burano interests you then I say go- we visit every time we are in Venice- it is just so charming-
Get vapoertto passes for length of stay and you can go anywhere you like- just riding the vaporetto is fun!

On departure day you could visit Rialto fish market in the am. Doesn't take long, kids might like seeing all the interesting seafood.

you could visit Rialto fish market in the am

Good idea. If either of the kids is a bit squeamish you should be prepared for a fair amount of lovable and very fresh tasting fish and other denizens of the deep will still be moving around.

If your children are readers, they might enjoy this bookstore. My teen nieces loved it. https://www.atlasobscura.com/places/libreria-acqua-alta