If only two, I would get Resistance 2 and LittleBigPlanet, both titles can provide endless hours of fun. I would recommend playing Resistance: Fall of Man first, but if you can't be bothered catch up with the story by watching youtube videos of the cutscenes.
Resistance 2 due to great single player and 8 player co-op campaigns and other extensive multi-player options.
LittleBigPlanet due to level creation and endless user created content.