The group $GL_n(\mathbb{F}_p[x])$ acts on the Bruhat-Tits building for $GL_n$. The vertex set is $GL_n(\mathbb{F}_p((x^{-1})))/GL_n(\mathbb{F}_p[[x^{-1}]])$, and the higher simplices form sets of the form $GL_n(\mathbb{F}_p((x^{-1})))/I$ for various parahoric groups $I$. The action of $GL_n(\mathbb{F}_p((x^{-1})))$ on the left restricts to an action of $GL_n(\mathbb{F}_p[x])$.

When $n=2$ there is a nice exposition of the action in chapter 2 of Serre's *Trees* (as long as you're okay with $PGL_2$ instead of $GL_2$). You get an infinite sequence of 1-simplices that vaguely resembles the $SL_2(\mathbb{Z})$-quotient of the complex upper half-plane. In higher rank, I don't think things can be quite so explicit.

You can also find actions of $GL_n(\mathbb{F}_p[x])$ on many other spaces, namely those that are defined over rings containing $\mathbb{F}_p[x]$ and that come equipped with a natural algebraic or analytic action of $GL_n$. For example, the analytification of the flag variety for $GL_n$ over the completion of an algebraic closure of $\mathbb{F}_p((x^{-1}))$ admits such an action.