I have just installed Civ 5 from Steam. Is it possible to play it offline (without connecting to the Internet?) If yes, How?
OK. I intalled the game myself and did a bit of testing. First of, you'll need to run the game once while the connection is still on, to complete all the installations and such. You don't need to play the actual game. Just exit once you reach the menu. (You might even be able to get out during the cutscene by pressing Alt+F4.)
After this, open the Steam window, go to File (upper left corner) and choose "Go Offline...". Steam will restart in offline mode (or at least shut down and be in offline mode once you start it again) that allows you to play the game offline. The game itself doesn't seem to have any additional need for internet connection.
However, this might come a bit late, since the free weekend is about to end. The game will most likely disable itself once the event is over, wether you're online or not.