We have an extra day in Bologna and are thinking of a day trip to Venice. (Bologna to Venice and back in the same day) Is this advisable?
It's totally feasible if you really want to see Venice. The fast train takes only 90 minutes between the cities so you can leave at 9 the morning and return on a 7:30 pm train and have a whole day there. If you book in advance you can get "super economy fares" for a very reasonable price. Even at full price it will only cost about 30 euro each way. I'm not a Venice lover, but feel you should see it once in your life. You can at least see the city and get a taste for it in one day.
Venice lover here. If you can do an overnight in Venice you wil enjoy it about 10x more than just daytripping in. If not, a daytrip is better than not seeing it. Do get away from the San Marco/Rialto corridor to see more of that amazing city.
Go for it. Many years ago we did a day trip from Verona, being so close by we could not resist. I actually gasped when we walked out of the railway station, it was like walking into a painting (I was younger and more impressionable then!) Since then we have been back seven times for a few days holiday at a time and thinking of going again this December. Venice is not for everyone but be warned you could get hooked!