I second the recommendation for Natsume Yujincho ('Natsume's Book of Friends'). Girls seem to love it, on the Japanese audience side at least. A sadboi goes around solving ghost problems (a lot like Mushishi) while experiencing pathos and learning the value of family and friends along the way. Apparently the mangas are mostly translated now but all I've seen is the anime, which is certainly good. Personally I rate it a bit above Mushishi, which is too dragging & contemplative for me.

If I were a cool nerd uncle I'd probably just rip them all off 9anime and stick them on a thumb drive for the kids to watch, but maybe you're the kind of cool uncle who stays within the law.

I don't know if they'd be into it, but my own favourite pick for a slower, sedate story which could(?) be enjoyed by girls is Uchouten Kazoku ('The Eccentric Family'). It's another mix of folklore and slice of life set in Kyoto, based on works by the same author who did Tatami Galaxy and The Night is Short, Walk on Girl, and shares vibes with those anime, if you're familiar, but it's somewhat less surreal and out there. Don't be misled by what seems to be a tranny/trap-fuel first couple of episodes.