Is it possible to make a day trip to Venice from Innsbruck? If so, how and if not, what would be the best place to go other than Munich and Salzburg?

Innsbruck is awesome--I say stay put and/or enjoy some of the nearby small villages.

I think you might find Venice a bit far. By train, it would be minimum 5 hours, 1 way.

I think the time for a trip to venice from Insbruck is more like 5 houea 15 minutes to the Mestre station and then it takes a few more minutes to get to the center of the action.
Then you have over 5 hours back.
Not much of a day trip in terms of payoff at the main attractions.
Even if you left at 5:49 in the am you would be noon getting there on that particular train.
A 7:07 departure puts you back in Innsbruck at 12:30 or so in the morning.
I am not sure it is worth it.

When I was planning a family vacation in Innsbruck, I received info about day bus excursions to Venice, so I assume the reverse trip could be made. I think the bus left Ins. around 6.am and returned late at night. Long, but doable!

Putting it bluntly, a day trip from Innsbruck to Venice just doesn't make sense. Assuming you want to go south into Italy, you might want to consider Bolzano, Merano, or even Cortina. They will give you a different experience and aren't so far away. Good luck.

Not reasonably. Either stay in Innsbruck or give Venice the time necessary to enjoy it (a minimum of 3 days). You would spend the entire day getting there & returning.