Can I download an upcoming game and then activate it on the release date?
This depends entirely on the game. Steam refers to this practice as 'pre-loading' and they do it for select titles. In the specific case of Civ V, yes, you can preload. For other titles, you can check that games respective forums at the Steam community site to get the answer in most cases.
As far as i know Steam is already pre-loading this game, however not all games can be pre-loaded on Steam.