Galactic Adventures adds a tremendously rich and entertaining new layer to the Spore game.
Playing missions is only half of the fun of the game. The expansion also provides a powerful new adventure creator tool. It's a complex tool that can seem a bit daunting at first, but after a bit of practice (and some helpful tutorials) you get the hang of adventure creation. Making adventures with your creatures is a blast. As far as enhanced gameplay is concerned, I'd say that this is by far the best expansion Maxis and EA have come out with for any of their games.