Beat the game with any character on single play showtime mode and get all Platinum records. You'll get:

Hidden characters:
The Dude
Helga
Sorrow
Vox
Joe

Alternate Costumes for:
Ishani (Business outfit)
Ishani(Retro silver foil outfit)
Dwayne (Mullet outfit)
Dwayne (Purple and green superhero outfit)

Unlockable Venues:
The Garage
The Wagon Wheel
HMX Records

Unlockable Songs:
Rich Girl
I've Got You Under My Skin
Virtual Insanity

Unlockable Video Extras:
Wavegroup sings
Harmonix Sings
Playtesters Sings
Konami Sings
Wavegroup