This is the 12150th most frequent Polish word.
malowniczej
Picturesque.
Here, 'malowniczej' is used to modify the genitive form of 'doliny' (valley), indicating its scenic aspect.
Moje mieszkanie ma widok na szczyty malowniczej doliny.
My apartment overlooks the peaks of a picturesque valley.
The word 'malowniczej' describes the genitive form of an adjective that conveys a picturesque or scenic quality about a noun, in this case, 'rzeki' (river).
Spacerowaliśmy wzdłuż malowniczej rzeki.
We walked along the picturesque river.
In this sentence, 'malowniczej' is used in an instrumental sense to describe a feature of the 'okolicy' (area), relating to its stimulating scenic beauty.