There a 2 hidden bonus levels in the game, Eggipus Rex and Hot Coco.
To get to Eggipus Rex you need to go through the secret exit in Dino Might. Yellow Gem required. This will get you the Getting Carried Away trophy.
To get the gem in Eggipus Rex you just need to ride a dinosaur to the end. No checkpoints but the level is not too long and there are lots of Wampa fruits on the way.
To get into the hidden Hot Coco level you need to go through the secret exit in Road Crash.
In this level you play as Coco. Your goal here is to smash all crates. No time limit, none are missable. Hitting checkpoint boxes will save your progress if you die. Make sure to only hit them after you collected some hard crates. Watch out for bombs and sharks. Best to hit the ! block early on to destroy the nitro crates. After all 70 crates are hit head back to the start location for the gem.