I borrowed this anthology (possibly with "2000" in the title) from the young adult section of my local library in the UK in probably the mid 1970s. There was a specific adult science fiction section in the library, so I am presuming it was written for young adults.
The main story I remember was possibly called "The Zoo". A group of school children are wandering around a zoo and looking at an alien in a cage. At the end of the story the viewpoint is switched and the alien is a human astronaut who has returned to Earth to find it is far in the future. He wonders how it is that mankind has changed so much in the interim, as they now have three legs.
There was another story in the anthology about a man meeting the devil and going down in an elevator. Within it is the old adage, "if you eat with a long spoon you will sup with the devil", but it's not the Wyndham story and I can't remember any more detail about what happens to him.