Mario Kart: Double Dash / Codes and Tricks

Unlockable Karts
Barrel Train - Finish Star Cup in 150cc
Bloom Coach - Finish Flower Cup in 50cc
Boo Pipes - Finish Special Cup in Mirror Mode
Bullet Blaster - Finish Special Cup in 50cc
Green Fire - Finish Mushroom Cup in 50cc
Para Wing - Finish Star Cup in 50cc
Parade Kart - Finish All Cup Tour in Mirror Mode
Piranha Pipes - Finish Star Cup in Mirror Mode
Rattle Buggy - Finish Mushroom Cup in 100cc
Toadette Kart - Finish Mushroom Cup in Mirror Mode
Turbo Birdo - Finish Flower Cup in 150cc
Waluigi Racer - Finish Flower Cup in 100cc

Unlockables (other)
All Cup Tour - Finish Special Cup in 150cc
New "Thanks for Playing" Screen - Earn Gold for all race cups & classes
New Start Screen - Earn Gold for all race cups & classes
Mirror Mode - Finish All Cup Tour in 150cc

Staff Ghosts
To unlock the staff ghost for each time trial track, beat the following times.

Baby Park - 1:14.000
Bowser's Castle - 2:47.000
Daisy Cruiser - 1:55.000
Dino Dino Jungle - 2:03.000
DK Mountain - 2:15.000
Dry Dry Desert - 1:53.000
Luigi Circuit - 1:29.000
Mario Circuit - 1:44.000
Mushroom Bridge - 1:34.000
Mushroom City - 1:53.000
Peach Beach - 1:23.000
Rainbow Road - 3:19.000
Sherbet Land - 1:28.000
Waluigi Stadium - 2:02.000
Wario Colloseum - 2:24.000
Yoshi's Circuit - 2:02.000

Other Tips, Tricks, and Secrets

Mushroom Cars
Run into the cars that look like mushrooms, and they will throw out a mushroom for you to use as a boost.

Start Boost
At the beginning of a race, accelerate right as the light turns green, and you will start with a boost. Practice makes perfect for this!

Lakitu Boost
When Lakitu picks you up and puts you back on the track, accelerate right before he puts you down and you will get a turbo boost. It will take some practice to time it correctly.

Japanese Easter Egg
After completing a Time Trial race (When it says Retry, Change Track, etc,), press L,R,L,R,X,Y,X,Y,Z. The code that pops up was for a Japanese contest.