These are very obviously "hidden," but here are some off the top of my head:
In the "Psychedelic" level, head as far left as you can and you should see a bit of level sticking out to the left on the map. Drop down into the pit and hold left to fall through the wall there and it'll take you to a present.
In the cave level, don't remember the name, there's a present right to the left of the giant bug you need to lead away to the right.
In the Cute level, head as far right as possible on ground level and you'll see a bunch of drillable, brown rock. Drill right and you'll an icon flashing in the wall, drill there to find a present.
In the Club level, you'll open up a larger area up top where you ride a motocross wheel. Use the launchers on the far right of this room to get as high up as you can in the room, then boost jump to the left to land on a roof with a present on it.