Ok, I'm not crazy about suggesting you what you already know: Venice is unique, impossible to miss, a real must for anybody.
When you do go to Venice though, please consider it's very very very touristic, so you might want to visit it a bit off season, that is, not in December before Christmas, and especially not during summertime. Too hot and humid.
Despite the rumors that claim Venice smells, I believe it doesn't smell at all. Obviously if you go in the middle of heat you can't expect to find it refreshing.. so again, don't go in july-august.
When you go to Venice, please don't hang around only in Piazza San Marco, but explore as much as possible. One touristic thing you can do without feeling guilty though, is to send a postcard from Venezia, the one and only.