Ich mache nichts Besonderes.

Like English we have two similar verbs: to do (tun) and to make (machen).

However in English it is more common to do things. To make is mainly used in the context of producing something. (cake, homework, chairs...)

In German it is the opposite. We prefer machen for both meanings, and use tun only in certain expressions.

Here Besonderes is a nominalized adjective

In other words, it's the adjective besondere turned into a noun