Learn Russian: Твой ум меня удивляет: ты можешь использовать абсолютно любой простой материал! - Your mind surprises me: you can use absolutely any simple material!

In Russian, любой means 'any' in the sense of a free choice: whichever one you pick. всякий leans more toward 'every kind of' or 'all sorts of'. Because the sentence emphasizes that you can take absolutely any random item from a limitless selection, любой is the right fit. Also, абсолютно любой is a very common fixed phrase, whereas абсолютно всякий sounds unnatural.

Isn't любой простой материал the direct object? Why does it look like it's in the nominative case?

It is the direct object, so it is actually in the accusative case! Remember that for inanimate masculine nouns like материал, the accusative form is identical to the nominative. The pronoun любой and the adjective простой simply agree with this inanimate accusative form.

Why do we use использовать here? Is it imperfective or perfective?

использовать is a 'bi-aspectual' verb in Russian, meaning it functions as both imperfective and perfective depending on the context. Here, because it describes a general, ongoing ability rather than a single completed action, it is acting as an imperfective verb.

Why is the object pronoun placed before the verb in меня удивляет?