Miyori no Mori

Both are movies about a young girl who's forced to go to her grandparent's during the summer, but doesn't want to because they live in a remote place. However, they rejoice after finding fantastic creatures and make friends around town.  

Both A Letter To Momo and Miyori’s Forest feature a female protagonist, both of which initially have similar personalities. Both protagonists move to rural Japan to live with older relatives after the absence of a parent. While both are G rated supernatural dramas, Midori’s Forest focuses on environmentalism and spirits and A Letter To Momo is centered around inner conflict and youkai. Both protagonists use spirits and youkai to accomplish their goals in these two coming of age movies.  

Both movies involve the relationship between humans and forest spirits, and both have a good dose of environmentalism built in to the plot. Also, both are aimed at younger audiences. 

Both movies have a similar premise - a human girl working with spirits to protect the forest from destruction by "greedy" human developers. Miyori no Mori is like a sanitised version of Mononoke Hime that is meant for younger audiences.  

These two are very similar. Both revolve around animals trying to scare people away from destroying their habitat, at least in the final half with Miyori no Mori.