"Mayohiga": An illusionary house that bestows wealth upon lost travelers, Unlike houses in the mysterious and slightly eerie stories of legend, in this modern retelling, Mayohiga is a cafe staffed by beautiful Onee-san.

What are the best anime series that aren’t about high school?

My first choice would be Fullmetal Alchemist : Brotherhood. It is beyond brilliant.
The storyline and the characters are explored to an extent that even the most popular TV series and movies don't have.
Graphics are brilliant, the plot is amazing, and the whole concept of Alchemy and Equivalent Exchange are very innovative and unique.
To be honest (I might get a bit of flak for this), I would rate FMA:B above Death Note in my list of favourite anime. And, although the other answers have already mentioned this, I'd like to emphasize this one: Steins;Gate. Unlike most anime, it has a very small set of characters.
Basically just the 8 shown in the picture.
But the series' exploration of the concept of time travel is surprisingly good.
When I started it, I expected it to be some cheesy show about kids who mess around with time travel.
But it's more...
Some serious effort has gone into this show, even thought it's just 25 episodes long.
Oh, and most of the main characters are of high school age, but that doesn't hinder the show in any way.
In fact, it makes it easier to connect to the characters. Thanks for the A2A!