Prebook online so you can just turn up at your alloted time slot and therefore skip the queues! You can pay to just enter the basilica and then it is extra for the audioguide and more again to go up one of the towers. To be honest, the tower was a bit of a waste - other than the view you don't get to see much else so this is probably the bit to drop. The audio guide is worth the extra as it details info as you walk round which gives you a real sense of place. This is probably the one must do in Barcelona but definately prebook!