Translation:He is interested in making wooden things.
Since robienie is a noun, let me rephrase the English sentence in terms of a more nouny noun:
He is interested in construction of wooden items.
Notice the preposition of. Almost all of its uses in English, including this one, are performed in Polish by genitive.
It's because of "robieniem" I think the rule after this kind of noun is like with negation. (accusative->genitive)
Precisely so. The grammar term is "przydawka dopełniaczowa"(genitive attribute), which is used here to describe the gerund "robienie"(in instrumental).
As the name implies, this kind of attributive is always in genitive(cf. "Jego interesuje robienie drewnianych przedmiotów").
is it genitive all the time or like negation, affects only accusative? I'm pretty sure it's kierowanie ruchem (instrumental), and pomaganie biednym (dativ)
Przydawka dopełniaczowa is always in genitive(that much I remember from język polski in school ;-) ), but there are many other kinds of przydawka. I think(but I'm not sure, to be honest - I don't remember that much from j. polski) your examples are of przydawka rzeczownikowa kind(which is created from noun in any case other than genitive, if my memory serves).
Also, I think "kierowanie ruchem" might be a set phrase, because "Policja przejmuje kierowanie ruchem gdy nie działają światła" but "Rolą przydrożnej reklamy jest kierowanie ruchu na stację paliw".
Seems a bit colloquial to me, but maybe I exaggerate... ok, let it be, added.
I used "items" in the SAME sentence in this lesson, and it was correct then, but not now.
"items" is accepted, should have worked, it must have been a bug. Sometimes Duo rejects a correct answer :/
Now i typed the EXACT translation but it's still "wrong" and I'm stuck at this sentence. ???????