The latest Humble Bundle offers the chance to own three complete Civilizations, as if you were trying to recreate the British Empire. Along with Civs III, IV, the lowest tier contains the recent Ace Patrol games, which are fairly lightweight, but clever, replayable and oddly adorable considering the World War dogfighting subject matter. All of that, along with the latest incarnation of Railroads! can be yours for any chosen price. Pay more than the average (at time of writing, $8.16) and you'll receive Civ V, and the Gods and Kings DLC. More than $15 unlocks the Brave New World DLC as well. BONUS: approximately two days of a Humble Paradox sale remain.
I have Civs IV and V installed at all times, and they happily co-exist. Two very different approaches to a common theme. I haven't played III for ages and I'm sure someone will argue that it's worth going back to, but I've always thought of II and IV as the high-points of the series, with V having grown into a pleasant diversion.
The main draw of the bundle, since it lacks Covert Action for which you'll have to visit GOG, is Ace Patrol. If you haven't played the turn-based aerial combat game yet, this is a fine opportunity to correct that error.