We were there in summer 2003; walked the feet off the first day, so spent most of the 2nd day in water taxis (vaporettos) chugging around everywhere, Murano, Burano, the Lido. 10 euros bought you a ticket to cover all the buses and water-buses (per day)
Verona's not too far away: we went for the opera in the Roman amphitheatre: as my husband professes not to like opera,we went for "Aida" which was totally spectacular, and almost converted him (apart form long aria/dialogues in Act 3) But not sure if the season for that is finished now.
Agree - go for any concerts in churches, esp Vivaldi or Gabrieli - and if Banditaliana (Ricardo Tesi) just happens to be in town you've got it made! If you can afford a gondola, they will sing for you - or you could sing for them!