There are two easter eggs to be found in this level; one requires you to fly high, while the other requires you to fly far.
Egg 1 - Go to the 4th checkpoint. Get the afterburner and fly up to the second platform, which is located dangerously close to the roof. Slow down as you approach the next jump and an afterburner will fall from above. Use it to fly up though the gap in the ceiling and turn right to get into the next room. Drive forward and you'll find a bunch of squirrels tumbling down a slide.
Egg 2 - Get to the final checkpoint. Get the afterburner and fly forwards, but point yourself upwards. Try to land on the roof of the finish platform, then drop off the back side of the roof. Jump over the next gap and drive to the end of the hallway. You'll find a pit at the end of the hallway; stop and wait for a set of explosive barrels to roll into the pit and detonate, then drive in. You'll find a few squirrels hidden among the scaffolding and other objects.
They're just squirrels. Interestingly, though, they were used in place of easter eggs that BlueMageFTW had originally put in his Big Thrills track for the Big Thrills contest.
This section is reserved for Confirmation/comments about this investigation. Investigations will remain open until advised by Redlynx that the final solution for each secret is found.