favorite Venice day trip?

Murano, Burano and Torcello Or Padova (Padua) Or Vicenza (Actually you might be able to squeeze both Padua a Vicenza in a busy day) Or
Verona

All of Roberto's suggestions are excellent. My favorites are Padova for the Giotto frescoes at the Scrovegni Chapel ( get your ticket as soon as you can), and the Chiesa devil Eremitani very nearby. Trains run from Venice frequently, it takes about 30 minutes to get there. Tortellini has some beautiful mosaics and if you have a vaporetto pass, you can use it to get there.

Tortellini has some beautiful mosaics and if you have a vaporetto pass, you can use it to get there. Autocorrect, Zoe? Torcello

Trieste and Ravenna

And for more 'tortellini'...;-) BUT...consider yourselves warned...it's quite racey... I LOVED the Scrovegni Chapel in Padua/Padova; they were offering the 40-min long sessions (normally 15(?)mins), and it still wasn't enough ;-) But I'm a Giotto/frescoes/mosaics freak...YMMV.

I'll agree with Murano, Burano and Torcello , but I spent so much time in Murano.....I went back a second day to spend more time in Burano. Also enjoyed the cemetario island - very interesting looking around.
Saw Ezra Pound's grave and several other notables.

Thanks, guys, I meant Torcello, but forgot to turn off the auto spell-check.

All of the above are excellent suggestions. Read up on them to see which ones interest you most. None of them require either a car or advance reservations (except for the Scrovegni Chapel - fantastic!) I just want to second the recommendation for San Michele, the cemetery island of Venice. It's not really a day trip (it takes only a few hours at most), but it's fascinating. Be aware that there is no shade; on the day I went, I wished I had an umbrella, even though there was no rain. The vaporetti continue from San Michele to Murano, so you can combine these two easily.

...Saw Ezra Pound's grave and several other notables.... Yes , Diaghilev and Stravinsky !!

We loved our day trip to Verona. Great historical city that's not too far by train. Awesome Roman Colosseum that is still in use. Great restaurants and a cool open air market. We make it a point to visit the city every time we return to Venice.

I loved Burano and Torcello - in Murano they just want to sell you glass. It's easy to catch a vaparetto from Venice proper to all the islands in the lagoon.
I also highly recommend Verona - easy to get to via train and spectacularly preserved and beautiful! Go see an opera in the Roman collesseum in Verona - even if you're not an opera fan it's an amazing experience!