Here's the Shell Cup in 150cc. It's brings back some classics, and since it's the first Retro Cup, it's not too hard, especially with SNES Ghost Valley 2 possibly being the easiest course in the Grand Prix thanks to a shortcut the CPUs never use.
Tracks:
GCN Peach Beach
DS Yoshi Falls
SNES Ghost Valley 2
N64 Mario Raceway