We walked everywhere. It was the best thing to do - seeing the less crowded bits of the city was wonderful. The obvious bits are absolutely chockablock with tourists ( I think the number of cruise ships that calls in has something to do with that) so you have to grit your teeth and not be in a hurry. It's all worth seeing, of course, but you just have to be prepared for the crowds.
Stuff what I especially enjoyed:
The Jewish Museum, which is small but very well done, with their tour of the local synagogues- really, truly fascinating.
we did a free walking tour on our first day which was ideal for us, as I'd only been before for a day when I was 14 and my friend had never been. They steer clear of the main attractions, and give you a really good walk round some of the other districts. You don't pay up front; you just tip after. We thought it was excellent http://www.venicefreewalkingtour.com/
The other thing we did was a sort of behind the scenes tour of the Doge's Palace. I think it was 19 euros each, but it includes entry to the main bits of the Palace anyway ( which is about 14 euros). It was fascinating- covering the prison, the mechanisms of the state, and Casnova's imprisonment and escape. http://palazzoducale.visitmuve.it/en/the-museum/layout-and-collections/secret-itineraries/
If you're looking for accomm, we stayed here, and it ticked all the boxes. Just a minute away from the boat stop where the airport boats stop, plus the normall vaporetti, a decent Co-op less than 10 minutes away, in a really interesting, not too touristy area, good cafes etc round the corner, and 20 minutes ace walk to St Mark's Square etc. https://www.airbnb.co.uk/rooms/9039288
Oh! and soft drinks are more expensive than booze. I became very partial to an Aperol spritzer.