There was a collection of sci-fi/fantasy short stories I had as a kid. I can't remember the names of any of the stories but I can remember some of the contents well:
One story was about some tiny aliens whose spaceship crashlanded on Earth and they have to survive the perils of a backyard or something similar. I think they all ended up dying by the end.
Another story was about a school where kids have to grow their own tiny star as part of a class project. One kid screws it up and creates a black hole that destroys the Earth.
Another story was about a kid who stumbles into some ancient machine that aliens left on Earth millions of years ago. He had to solve an intelligence test by solving a puzzle related to Earth's place in the solar system. He saw some other lifeforms from Earth's past that had already failed, I think including a caveman, a pterodactyl, and a giant centipede or similar.
There was one I only vaguely remember about a planet that was 99% water with only a few islands. I think some astronauts crashed there and had to find a way off?
I believe it included a chapter from Ursula Le Guin's "A Wizard From Earthsea," I think it was the one where Ged faces down a dragon. I remember that it was completely understandable by me at around the age of 10 or so.