13 hidden characters, 9 hidden karts, 9 hidden bikes, other unlockables, and how to get them.
Obviously, spoilers below…
Starting Characters
Baby Mario
Baby Peach
Toad
Koopa
Mario
Luigi
Yoshi
Peach
Wario
Waluigi
Donkey Kong
Bowser
+ enlarge, photo by SpikeyNDS
Unlockable Characters:
King Boo - Win 50cc Star Cup
Diddy Kong - Win all 50cc Cups
Baby Daisy - Attain at least 1 Star Rank for all 50cc Wii Grand Prix Cups
Mii (Outfit A) - Win 100cc Special Cup
Mii (Outfit B) - Unlock 32 Expert Staff Ghost Data in Time Trials
Mii (Outfit C) - Get 15,000 points in VS mode - *unconfirmed
Dry Bones - Win 100cc Leaf Cup
Bowser Jr. - Attain at least 1 Star Rank for all 100cc Retro Grand Prix Cups
Daisy - Win 150cc Special Cup
Dry Bowser - Attain at least 1 Star Rank for all 150cc Wii Grand Prix Cups
Funky Kong - Unlock 4 Expert Staff Ghost Data in Time Trials
Baby Luigi - Unlock 8 Expert Staff Ghost Data in Time Trials
Birdo - Play Time Trials on 16 different courses/Get 250 wins online
Toadette - Play Time Trials on all 32 different courses
Rosalina - With a Super Mario Galaxy save, win a series of Grand Prix cups/Without Galaxy save, Attain at least 1 Star Rank for all Mirror Grand Prix Cup
Unlockable Karts:
Cheep Charger (small kart) - Attain at least 1 Star Rank for all 50cc Retro Grand Prix Cups
Blue Falcon (small kart) - Win Mirror Lightning Cup
Rally Romper (small kart) - Unlock 1 Expert Staff Ghost Data in Time Trials
Turbo Blooper (medium kart) - Win 50cc Leaf Cup
Royal Racer (medium kart) - Win 150cc Leaf Cup
B-Dasher (medium kart) - Unlock 24 Expert Staff Ghost data
Piranha Prowler (large kart) - Win 50cc Special Cup
Dragonetti (large kart) - Win 150cc Lightning Cup
Aero Glider (large kart) - Win all 150cc Retro Grand Prix Cups
Unlockable Bikes:
Quacker (small bike) - Win 150cc Star Cup
Bubble Bike (small bike) - Win Mirror Leaf Cup
Magikruiser (small bike) - Play Time Trials on 8 different courses
Rapide (medium bike) - Win 100cc Lightning Cup
Nitrocycle (medium bike) - Attain at least 1 Star Rank for all 100cc Wii Grand Prix Cups
Dolphin Dasher (medium bike) - Win Mirror Star Cup
Twinkle Star (large bike) - Win 100cc Star Cup
Phantom (large bike) - Win Mirror Special Cup
Torpedo (large bike) - Unlock 12 Expert Staff Ghost Data in Time Trials
Misc Unlockables:
Picture option to send to Wii Message Board - Complete all 8 Cups in a class
Karts & Bikes (50cc) - Earn trophies in all 50cc Grand Prix Cups
Karts & Bikes (100cc) - Earn trophies in all 100cc Grand Prix Cups
Mirror Class - Win all 150cc Cups
Alternate Title Screen - Win all Cups
Expert ghosts
Defeat a normal course ghost with a time at least seven seconds faster to unlock the expert ghost for that course.
Alternate title screen and ending sequence
Get a “One Star” rank or better in all Cups, including those in Mirror mode to unlock an alternate title screen and ending sequence.
Special credits
Win all trophies in a class to view the credits and see a congratulations screen featuring your Mii with Mario and Peach. Win all Grand Prix Cups to get another congratulations screen featuring your Mii with all the racers.
Quick start
When the countdown starts, wait until the “2″ begins to disappear, then hold 2 to get a boost at the start of the race.
Quick recovery
When you fall into an obstacle, press 2 precisely when you land to get a temporary boost.
Profile name stars
You can add a star next to your profile name on the results screen and the Wi-Fi races by completing all Grand Prix races with a certain rank:
One star profile name: Earn a “One Star” rank in all Grand Prix races.
Two star profile name: Earn a “Two Star” rank in all Grand Prix races.
Three star profile name: Earn a “Three Star” rank in all Grand Prix races.
Alternate Title Screens:
How to avoid the POW block:
Subject: How to unlock EVERYTHING 
