Fly or Train into Venice

Honestly, if you're only going to spend one night in Venice, then I wouldn't bother. Especially if that night is your first night after a long haul flight, and that day was a jet lagged blur. That's a guaranteed recipe for hating Venice. If you are flying in to Venice, try hard to spend at least 2 nights there.. If you must spend only 1 night from Florence, dont do it on your arrival day. Otherwise, just enjoy Florence and the rest of Tuscany.

This will probably be our only trip to Italy. Should we at least do a day trip to Venice from Florence?

Also we are considering flying in or out of Bologna- that might make an easier day/overnight trip to Venice?

Venice does not work well as a day trip. Streets full of water etc.

You might want to check into flights to Milan and out of Florence. Trains to Venice are frequent enough and quicker than from Rome. Day trip to Venice? Not my idea of time well-spent. Venice really could use at least 2 or 3 nights. Venice in the evening hours is very much different than during daylight! Florence is a few hours away by train but trains are frequent. I usually plan to leave Venice at noon and get into Florence in time to check in to my VRBO apartment. From Florence, day trips to Lucca and Siena are easy and worthwhile. Honestly, I prefer to stay over night at any town I visit. Day trippers are gone and the real 'culture' appears. Besides, dinner isn't until at least 9 o'clock and could easily last for a few hours. Ah, Italy!

I agree with CJean, one night is not worth the trouble. The time spent getting to and from the airport/train station to your hotel could eat up a good chunk of time, and you won’t get a good impression of the city. I would suggest you need a minimum of two whole days on the ground, which would mean three nights.

Taking the train in Italy is much more enjoyable in my experience.

As CJean indicated, one day in Venice is not doable. That's like saying one day in Paris, or London, or New York. It would be ridiculous. The biggest mistake rookie travelers to Europe always seem to make is trying to cram everything into a short time frame. Consequently, they learn close to zilch, experience high stress throughout their trip, and subsequently complain about how bad it was. Don't be that person. Scale it down, take time in a few select locations, and actually enjoy your trip while learning something about the local culture. Europe will still be there in the future.

I don't know how long your vacation is (you mention several days in Florence/Tuscany), but if you have more than 3 nights in Florence and maybe 3 more to roam around Tuscany, any extra nights you have you could spend in Venice. So for example if you have more than 8 nights available in Italy in total, you could fly to Venice, spend 2 or 3 nights in Venice, then go to Tuscany for the rest of your stay. Try to devote 3 nights to Florence before your flight home from Florence.

A day trip to Venice from Florence is possible (2h13 min by high speed train), but even if you leave super early in the morning you will have only a half day basically to visit Venice. The last high speed train back to Florence departs from Venice at 7:26pm.

you will have only a half day basically to visit Venice

And that time is the worst time in my opinion. The morning and the evening are the best times to be in Venice.