Venice is still hot in sept, when we went a few years ago at beginning of sept it was around 25-30 *c. people always say it is expensive but we found if you went off the beaten track down the back streets we found some little gems, family run restaurants that we're very cheap and the food gorgeous and not an English/Irish pub in sight plus you can hop on to a vaperetto (waterbus) over to the lido- a stretch of beach just across the lagoon, where there has been known to have the odd film star found on it. (and I think the Venice film festival is on around sept time). and if you search the net there are plenty of little family run b&b's that would look after you better than some 5* hotels. this site has a few you could try looking at if it takes your fancy
http://www.holiday-rentals.co.uk/. it has the added bonus of not having any lager louts plus we hardly saw any screaming kids running about either...which is always an advantage ;) Venice is one of the most beautiful places i've ever visited, a great place to lose yourself in , chill out or just people watch. me and the hubby are off to Barcelona in august for a week without the kids and after reading Beunchico's write up- I can't wait. Where ever you choose I hope you have a great time :)