I think 10 days in Venice and the close surrounding areas would be fun!
Padua was a very easy day trip for us by train, definitely recommended. Vicenza and Verona were nice as well, we did them by car though.
Since you are looking forward to the food experience, a couple of 'experiences' we highly recommend:
1) A Cicchetti tour with a guide - We did this our first or second day, which was great as we learned about the different offerings, and some 'off the beaten path' locations. We then made a point to look for new places to try out the rest of our stay.
2) Market shopping (near Rialto) / Cooking school. This was a wonderful day - spent an hour or two shopping in the market, then went back to the house / commercial scale kitchen to cook everything up! (We did it with Enrica Rocca).
Regarding your flight - perhaps this is not part of your question, but I do not see your origin? I assume SFO...
I would say if the flight you want is available now in saver Y, either book it now (if you don't mind two change fees), or wait a few days for your return to open up and then book the round trip flight (if you only want one change fee).
The only downside to booking two one ways is two change fees.
Then keep checking... Saver business may open up, there may be a fare sales, etc.
If something better does open up, you will be happy enough with the better option so it will hopefully take the sting out of paying the change fee...
Have fun!