If you're looking for manga similar to Kimi ga Wasureta Ookina Mono, you might like these titles.
A short manga about a girl that goes to live with her wacky grandfather. Her grandfather, an inventor of strange items, likes to try them out on Minori. Many escapades then ensue as Minori learns to cope with these situations day by day.
A pair of metalhead brothers were psyched. They made their onstage debut when disaster struck - one of them collapsed. So to recuperate they decide to rest at their grandfather's farm in the country, but now the farm might be turned into a golf course! What are they going to do, and what's to become of the love that they left behind in the city?
A boy returns to his family home in the country and strikes up an odd friendship with the young child who lives next door, Sora-san, who believes he is God. However, it is Plato, the boy's pet turtle, with whom Sora develops a strong bond. Plato believes Sora to be the classical Greek philosopher Socrates and calls her his teacher. Together, they ponder the mysteries and wonders of life in the quiet but beautiful countryside.
Wild, and bound by nothing, the large-framed young man Taiji returns to his home town for the first time in 10 years, where the main industry is dairy farming. In the mountain forest that is his inheritance from his grandfather, Taiji - with a sidelong glance for the dairy farmers around him, who blithely proceed to industrialize the act of obtaining the breast milk of the creatures known as cows - settles into his own pace to begin the process of setting up his own farm.