Any of the Little Big Planets are good. Simple enough for my girlfriend to play and enjoy but still challenging for someone who has played video games for years like myself.
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Another good one is an indie "game of the year" called Badland. Its another side scroller but like Little big planet it is simple enough for my girlfriend but still challenging at the same time. You can find it on PSN for less than $20 and it's worth every penny.
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Castle crashers is a classic couch co op. Old school arcade gameplay like the old ninja turtles game.
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All these games you can find under $20 on PSN and very much worth it.