Unlockables
Leaf Cup ... Achieve gold on Shell Cup and Banana Cup
Lightning Cup ... Achieve gold on Leaf Cup
Star Cup ... Achieve gold on Mushroom Cup and Flower Cup
Special Cup ... Achieve gold on Star Cup
More Kart Choices ... Achieve gold on all courses in 100cc, 150cc, or 150cc (mirror) Nitro Grand Prix
150cc Mirror ... Achieve gold on 150cc Retro Class (all courses)
Daisy & Dry Bones ...Achieve gold on all courses 50cc Nitro Grand Prix
Waluigi ... Achieve gold on all courses 100cc Nitro Grand Prix
ROB64 ... Achieve gold on all courses 150cc (mirror) Nitro Grand Prix. maintains beating just the Nitro cups unlocks ROB.
Final Mission ... Achieve one ranking on all courses for all missions one through six

Unlock Starry Names
To affix stars to your name in a wi-fi battle, just race all the grand prix races (including mirror tracks) and earn at least one star in the ranking. The number of stars earned during your grand prix career will be reflected in your wi-fi standing.
Vitto86
Unlockable: Alternate Ending Screen
You must complete all the classes with gold trophies on all cups, which includes mirror mode.

Hint: Blue Shell Block
First place racers can shoot a bomb backwards just before a blue shell hits to stop it from scoring a hit.

Hint: Burst Start
Hold A just after the two begins to disappear for an advantageous start.
roman960
Hint: Shroom Packing Time Trials
Any kart that has a full statistics meter in the items category will have 3 mushrooms in time trials.