Various unlockables:
To unlock more stuff, you basically only have to complete modes or songs.

Hard Story Mode - Beat Normal Story Mode

Very Hard Story Mode - Beat Hard Story Mode

EX Story Mode- Beat Story Mode on any difficulty

Note Pickup minigame - Beat EX Story on any difficulty as Luigi

Flight of the S.S. Brass minigame - Beat EX Story on any difficulty as Mario

Unlock 18 songs in Free Mode - Beat Story Mode

Unlock an additional 6 songs in Free Mode - Beat EX Story Mode

Unlock the song Choir on the Green in Free Mode - Play EX Story Mode on any difficulty and buy the Secret Song in the shop in World 1 for 100 Coins

Unlock the song Hop, Mario! in Free Mode - Play EX Story Mode on any difficulty and buy the Secret Song in the shop in World 2 for 200 Coins Unlock the song Wheres the Exit? in Free Mode Play EX Story Mode on any difficulty and buy the Secret Song in the shop in World 3 for 300 Coins

Unlock the song Piroli in Free Mode - Play EX Story Mode on any difficulty and buy the Secret Song in the shop in World 4 for 400 Coins

Unlock 10 minigames - Beat Story Mode as any character

Unlock the minigame Whee! - Beat Story Mode (Or EX Story Mode) as Luigi

Unlock the minigame Treasure Boxes - Beat Story Mode (Or EX Story Mode) as Mario

Unlock the minigame Coin Collection - Beat EX Story Mode

Super Hard Mode - Beat a song in Very Hard Mode without losing the dance meter