This was one of the best trips I've ever done. It was my first time in Italy and I got to spend 2 days and 3 nights in Venice. I stayed in Marghera district and took bus to Venice every day.
Make sure to bring comfortable shoes because you'll be walking all day long. There are no rental cars or taxis allowed on the islands. You'll have to see them by foot even though you might take a ferry to get to them. Make sure you'll buy a pass for public transportation as it's much cheaper then paying for ferries and buses (go to other districts) separately. I got 48-hour pass for under 30 Euros and used it about 20 times. If you purchase a single ticket for ferry, it's $6. You do the math!
In 2 days, I saw San Marco square, went inside of its Basilica (you have to check in your backpack and camera around the corner), on the top of San Marco tower (the best attraction b/c of amazing panoramatic views), took a tour of Palazzo Ducale (book behind the scenes tour online at least 2 weeks in advance), walked over Realto Bridge (too crowded and touristy), visited several churches, walked over many bridges... I also took a ferry to Burano, Murano, and Torcello islands. They were much more quiet and less touristy than Venice.
Don't do the gondola ride. It's $80 a absolutely touristy and crowded. You'll be waiting in line with other gondolas to pass certain canals and you might even get annoyed by drunk tourists on some of them. They are not romantic at all. And only very few guides actually sing like they show in the movies.
I highly recommend spending at least 2 full days in Venice. If you'd like to take it easy, sit in cafes, enjoy the food, live music, etc ., allocate more time. The food there is amazing and locals are very friendly. They understood my Spanish better than English.