There's not a lot especially close to Venice. You could go back to Florence. The one criticism I've read on this ABD is there's not enough time in Florence. That's a pretty easy train ride. You could do Milan, but I wouldn't recommend it with children. It's a cool city, great shopping/dining, but not a particularly fun city. You could spend some more time in Venice. We went to Padua to see Giotto's chapel, and Verona to see "Juliet's Balcony" and walk around. You could take the boats over to Murano. You could even go as far a Burano, but IMO it's not worth the farther boat ride. There's lots of great places to eat in Venice - see tripadvisor. There's also a lot of crumby places to eat in Venice. Don't just stop in the tourist traps. I think if it were 2 extra days, I'd probably stay in Venice. If it were 3 or 4 days, I'd probably either go back to Florence or over to Cinque Terre or do something totally different like head over to Croatia.