Topic: Super Mario Galaxy + Super Mario Galaxy 2

Once you get all 120 Stars you unlock a second campaign with Luigi (although it's just the same levels all over again just with you playing as Luigi other than Mario), then when you get 120 Stars AGAIN you unlock a secret final Galaxy called Grand Finale Galaxy with a 121st (although don't get too excited, it's just the Mushroom Kingdom tutorial level with Purple Coins everywhere, kind of an underwhelming final level). Only after you beat Grand Finale Galaxy will you 100% the game. Although you'd be forgiven for not wanting to do all of that, it is a bit much and there's not actually any genuine new content.