If you're looking for manga similar to Kyoukai Senjou no Horako-san, you might like these titles.
In approximately the year 3000, humankind arrived at the peak of its civilization, but is losing its will to keep living. It follows a girl, called "princess" by those around her, living on a small island with a fifty person colony and a shinning coral reef. She is descendent of Moon people, and it focuses upon how her ancestor supposedly came to Earth.
An unusual life of a grade school girl genius named Hatsume and her invention: a box robot called "Hako".