Racing Rewards in Mario Kart Double Dash:
To unlock everything, just win Gold on every Cup in every difficulty.

Waluigi Racer (Waluigi's Kart) - Finish the Flower Cup in 100CC

Special Cup (Wario Colosseum/Dino Dino Jungle/Bowser's Castle/Rainbow Road) - Finish the Star Cup in 100CC

Toad, Toadette, Toad Kart (Toad's Kart) - Finish the Special Cup in 100CC

Luigi's Mansion (Battle Mode Arena) - Finish the Mushroom Cup in 150CC

Turbo Birdo (Birdo's Kart) - Finish the Flower Cup in 150CC

Barrel Train (Diddy Kong's Kart) - Finish the Star Cup in 150CC

All Cup Tour (All 4 Cups in one) - Finish the Special Cup in 150CC

Mirror Mode (Selectable at the CC selection) - Finish the All Cup Tour in 150CC

Toadette Kart (Toadette's Kart) - Finish the Mushroom Cup in Mirror Mode

Tilt-A-Kart (Battle Mode Arena) - Finish the Flower Cup in Mirror Mode

King Boo, Petey Piranha, Piranha Pipes (Petey Piranha's Kart) - Finish the Star Cup in Mirror Mode

Boo Pipes (King Boo's Kart) - Finish the Special Cup in Mirror Mode

Parade Kart (Belongs to Toadsworth, but he doesn't race) - Finish the All Cup Tour in Mirror Mode

Alternative Start Screen - Earn Gold trophies for all cups in all race classes

Rattle Buggy (Baby Luigi's Kart) - Finish the Mushroom Cup in 100CC

Bullet Blaster (Bowser Jr.'s Kart) - Finish the Special Cup in 50CC

Bloom Coach (Daisy's Kart) - Finish the Flower Cup in 50CC

Para Wing (Koopa Paratroopa's Kart) - Finish the Star Cup in 50CC

Green Fire (Luigi's Kart) - Finish the Mushroom Cup in 50CC

Alternative "Thanks for Playing" Screen - Earn Gold trophies for all cups in all race classes

Japanese Contest code in Mario Kart Double Dash:
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. Totally useless, but it's interesting that they forgot to take that out.

Turbo Boost after Lakitu Retrieval in Mario Kart Double Dash:
If you fall off the track or get stuck underwater for more than a couple moments, Lakitu comes to pull you out and place you on the track again. When he picks you up, wait until just before you touch the track again, then press the acceleration and you'll get a turbo boost if you timed it correctly.

Mushroom City Quick Boosts:
As you may have noticed on the Mushroom City course, among the traffic are vehicles shaped/colored like the Mushroom items. If you run into them, you don't spin out like you would against other cars; rather a Mushroom is thrown from the back for you to collect/use for a quick boost!

Extra balloons in battle mode:
During a mushroom boost, or invinceability, ram into an opponent. Instead of them popping a ballon, you gain theirs. This does not work if you have three balloons, but they lose one normally.

Mario Circuit Goomba Boosts:
If the Goombas on Mario Circuit are hit with a turtle shell, they will be squished into the ground and a boost will fly off them. If a thunderbolt is used on the course, it will squish them all and there will be several boosts lying in that area. It doesn't work the same as the mushroom cars; if you hit a goomba they stay solid and you spin around a bit. Turtle shells are you're best bet.

Rainbow Road Falling Stars:
You may have noticed there are a lot of stars on Rainbow Road. And many of them fall. And some of those that fall fall on the track. Where those stars fall, they turn into star items, just sitting there. Some of these are dangerously close to the edge, but if you can get one (esspecially when you're already near the front and there's no other way to get one) it's a nice bonus. Only problem is that they're the same color as banana peels, so be sure before you run over something yellow!

Pipe Plaza Glitch:
In the balloon battle mode on the Pipe Plaza map. if an enemy hits you into a pipe with their weapon, and you are thrown through the pipe then you will not lose a balloon. Instead, you will come out of the pipe with stars around you and it will appear as though you just drove through the pipe.