"He is interested in making wooden things."
Translation:On interesuje się robieniem drewnianych przedmiotów.
I understand your logic, but it's more complicated. The gerund form "robienie" takes Genitive (you may understand it as "Making of wooden objects", and 'of' always suggests Genitive). So the noun phrase "making wooden objects" is "robienie drewnianych przedmiotów".
And then, and "being interested in sth" takes Instrumental, 'robienie' takes Instrumental, but it's still 'making of', so the wooden objects stay in Genitive. Thus, "robieniem drewnianych przedmiotów".
Generally (leaving some exceptions aside):
- adjective + noun is used when we have feature + noun. So: młody (young), piękny (beautiful), daleki (far) (Exceptions are like: Far East - Daleki Wschód, Upper SIlesia - Górny Śląsk)
- noun + adjective, is used when this combination defines some entity with feature that is not temporary or when we categorize. "Przedmioty drewniane" is something a shop that sells "wooden things" would write. Shops tend to place each adjective after the noun, like: „Przedmioty drewniane zdobnione”. In common speech the word „przedmioty” is so vague that I cannot think of it as of a category. Besides I can imagine that I would put each adjective „made out of X” like:drewniany, metalowy etc. before noun in normal conversation. However a juice „sok” is always like „sok jabłkowy” and nearly never like „jablkowy sok”.