To Unlock...                      You have to...
King Boo, Petey Piranha           Beat the Star Cup in Mirror Mode
Toad, Toadette                    Beat the Special Cup in 100CC
All Cup Tour                      Beat the Special Cup in 150CC
Tilt-A-Kart, Battle Arena         Beat the Flower Cup in Mirror Mode
Luigi's Mansion, Battle Arena     Beat the Mushroom Cup in 150CC
Mirror Mode                       Beat the All Cup Tour in 150CC
Special Cup                       Beat the Star Cup in 100CC
Alternative "Thanks" Screen       Beat the All Cup Tour in Mirror Mode
Alternative Start Screen          Beat the All Cup Tour in Mirror Mode
Luigi's Green Fire                Beat the Mushroom Cup in 50CC
Petey Piranha's Piranha Pipes     Beat the Star Cup in Mirror Mode
Toad's Toad Kart                  Beat the Special Cup in 100CC
Toadette's Toadette Kart          Beat the Mushroom Cup in Mirror Mode
Birdo's Turbo Birdo               Beat the Flower Cup in 150CC
Waluigi's Waluigi Racer           Beat the Flower Cup in 100CC
Diddy Kong's Barrel Train         Beat the Star Cup in 150CC
Daisy's Bloom Coach               Beat the Flower Cup in 50CC
King Boo's Boo Pipes              Beat the Special Cup in Mirror Mode
Bowser Jr.'s Bullet Blaster       Beat the Special Cup in 50CC
Baby Luigi's Rattle Buggy         Beat the Mushroom Cup in 100CC
Koopa Paratroopa's Para Wing      Beat the Star Cup in 50CC
Toadsworth's Parade Kart          Beat the All Cup Tour in Mirror Mode

Donkey Kong Mountian:
     When  you are in the Donkey Kong Mountian when you see the smilie
face  on  the  mountian,  tap  the  A button, the mountain smilie will
breath fire at the person behind you!

Steal items from other players:
     In  2/4  player  co-operative the person sitting on the back of a
kart  can  steal items from other racers. The driver simply has to get
up  close  next  to  somebody, (make both karts side by side) then the
partner  controling  the items at that point in time can press the R/L
triggers,  depending on which side the opposing kart is, then you will
end  up  with  the  other  karts  item. (if the other kart has no item
obviously you can't steal anything from them).