Venice is not necessarily an all indoors experience.  From your hotel, it's a brisk walk to [Piazza San Marco][1], and if you are up for a more arduous walk, check out the [Lido][2] (you'll need to take a ferry part of the way to get there). It's not too late in the season for swimming there and the beach is world class.

But most of all, September is a great time to use Venice as a base while exploring other islands in the lagoon.  [Burano][3], for example, is favoured by locals (and me) for it's authentic restaurants (along with less touristy prices).  

[Torcello][4] has a great open air market as does [Mazzorbo][5].  Mazzorbo has an artsy-craftsy fair which will still be open, and Torcello was the one-time home of Ernest Hemingway.  

All of the islands have well preserved architecture and several have ruins from the Roman era.  As an additional consideration, take note that during the high season (which encroaches into September), tourists outnumber locals by a [mind-numbing 600 to 1][6]! It makes getting away to the outskirts all that more attractive.


You can take ferries or private water taxis to any of the larger islands.  


  [1]: https://en.wikipedia.org/wiki/Piazza_San_Marco
  [2]: https://en.wikipedia.org/wiki/Lido_di_Venezia
  [3]: http://www.isoladiburano.it/en/index.html
  [4]: https://en.wikipedia.org/wiki/Torcello
  [5]: https://en.wikipedia.org/wiki/Mazzorbo
  [6]: http://edition.cnn.com/2015/03/30/travel/the-wonder-list-venice-bill-weir/index.html