The keys are not that difficult to find in this game but maybe you have trouble to find all of them.
Or maybe you are just to lazy 😉
its blocked by a wall but inside there is a bomb moster thing, just wait or spam tail whip
on the jump section with the icecubes jump left instead of right
at the portal section just fall down with the platform
bevor you dropp down to the platform with the lever, stay on the platform above it
after the section with the red, green, red blocks stay on ceiling and jump over the gravity switch
Now you have access to the bonus level.