Squirrel #15: Rocky Road
After passing under the conveyor belt on your way up several ramps, bailout backwards to launch yourself up to it.
Once you land on the conveyor belt, you'll be whisked away into the canyon below.
The next squirrel will then appear, as a rogue penguin wrecks the place.
Squirrel #16: Eco Park
This is another tough one. After clearing the first section of solar panels, bailout at the next jump and aim your rider at the cactus while holding Y/Triangle to grab it.
After holding on to the cactus for a few seconds, the screen should begin to flash different colours and you'll respawn in an hallucinogenic state.
You need to clear the rest of the track in a faultless run to reveal the next squirrel. If you crash at any point you'll have to restart the track for another attempt.
Squirrel #17: Cold Storage
After clearing the first jump and the tall obstacle beyond it, build up speed then bailout from the next ramp to launch yourself upwards.
If you got high enough you'll see a cable hanging down, which you need to grab by holding Y/Triangle.
This will pull you up to a platform where squirrels are busy digging away.