Ive seen the commercial daytrips advertised from Florence to Venice. Is it possible to take a train and just do teh trip yourself? The train tickets are less than half the price of the organised trips.
You can buy a ticket for yourself to take the train on a daytrip from Florence to Venice. The train terminates right there. The earlier you buy it the lower the price. It is about 2 hour and 30 minutes and very much worth it. I did that same day trip first time 16 years ago as a budget traveler first time in Italy. You really don't need a tour as Venice will be crowded with other tours and human traffic jams. Oh I think Rick Steves has a mp3 that you can listen to with a walk but It is enough just to walk see eat walk and see and take the Water Buses; or if you are a real high roller spring for a gondola ride with an opera singer. I recommend going to the top of St Mark;s Campinile for great 360 views over Venice. https://en.wikipedia.org/wiki/St_Mark%27s_Campanile
I did that last May on my birthday.
Do it yourself. 100%.
The trains from Florence to Venice are fast, frequent, economic and quite simply a pleasure to use. I would try and depart Florence as early as possible giving yourself the maximum time in Venice. You'll have 2 hours on the train to read about Venice and have breakfast (buy at one of the station bars before boarding instead of using the mediocre trolley service).
If you feel you are missing out on the organised tour/guide experience there is always the free venice walking tour which runs in the morning - I haven't used it myself but I am assuming you leave a 5-10 Euro tip at the end as nothing is really free in this life!
Better to spend the night in Venice and take in the city. You won't see that much on a day trip this this.
If you do just a daytrip to Venice you'll arrive with all the other hordes of daytripping tourists. You'll see the magical Venice emerge in the evenings after all of the daytrippers have departed.
I've done a day trip as well as 2-3 day trips. You can take the first train of the morning and leave by a late train if you don't want to stay overnight. I'd prefer to stay over because it's nice after most of the tourists leave and of course you have time to see more. .I'm a big fan of late afternoon/early evening trains to maximize sightseeing time. You could then stay over for one night, sightsee all day and take a late train back to Florence. Hope you have a great time no matter what you decide. (You probably know this but you can check out of your hotel in Florence, leave your luggage there, take a backpack then check back in. Every hotel I've ever stayed at in Italy has been happy to store luggage.)