Every time you see luigi you can paperize and grab him off the screen. When you do he runs away and in the round blue house in Decal Burg a newspaper article about luigi's appearance there is encounter. The newspapers are funny are essentially the luigi diaries of this game. Also if you find Luigi all 5 times he will leads the parade in the credits.
I've got a quote that embodies you perfectly, but it's seventy-three posts long, has a few massive flowcharts, and lots of Xion-KMA to Me
There isn't really a post-game per se, since you can't save after beating it. I mean, you can, but it's not like TTYD and SPM where it would actually be after it; Bowser still has Peach and there's no way to have a save file where Peach isn't kidnapped. If that makes sense.
As far as I know, the only thing that happens after you save after beating it is there's a Toad at the Sticker Fest grounds who lets you watch the credits parade again.
"An apostrophe does not mean 'uh-oh, here comes an s.'" - Dave Barry