I accidentally punched these on the ceiling while moving stuff around. They're around half a centimeter in diameter, and the ceiling seems to be plaster.
How can I mask/cover these up the most subtly?
Half a centimeter? In that case, spackle, sand, and paint. They make color changing spackle (pink to white) so you know when it's dry. Apply with a flat edge (e.g. a putty knife). Don't oversand, just enough to knock any rough edges off. And since the paint has an orange peel look, use a small roller for the touch-up.
Can't tell if it's a straight orange-peel or a knockdown texture. Either way, BMitch's spackle-sand-paint procedure is the way to go, possibly with a little extra texturing to make it not immediately evident there's been a repair.