Let $n\geq 2$. Are injective functions dense in $C([0,1]^n,\mathbb R^n) $ with the uniform norm?
No. Identify $\mathbb{R}^2$ with $\mathbb{C}$ and consider $f(z) = z^2$. If $g$ is close enough to $f$ then $\alpha \mapsto g(e^{i \alpha})$ stays in an annulus and winds around the origin twice, which cannot be done injectively.

$\begingroup$ Even simpler: use $f: [0,1] \to \mathbb{R}$ defined by $f(x) = x(x1)$. $f(1/2) = 1/4$, $f(0) = f(1) = 0$. If $g$ is continuous and within $0.01$ of $f$, then $0.01 \leq g(0) \leq 0.01$, $0.01 \leq g(1) \leq 0.01$, and $0.24 \leq g(1/2) \leq 0.26$. By intermediate value theorem, $g$ must attain the value $0.2$ twice, once in each subinterval. This seems like it might have been an undergraduate real analysis homework problem :(. $\endgroup$ – Steven Gubkin Apr 22 at 12:44

3$\begingroup$ To be fair, the OP specified $n\ge 2$ $\endgroup$ – Alessandro Della Corte Apr 22 at 12:44

$\begingroup$ @A.DellaCorte Ah, overlooked this. Martin probably has the simplest example then. $\endgroup$ – Steven Gubkin Apr 22 at 12:46