Can some finite group $G$ embed by conjugacy into $\operatorname{Sym}(H)$, for some $H \lhd G$?
What I could work out is the following.
Let $G$ be a finite group and $H \lhd G$. Then, the map $\psi: g \mapsto (h \mapsto ghg^{-1})$ is a homomorphism from $G$ to $\operatorname{Sym}(H)$, with $\operatorname{ker}(\psi)=\{g \in G \mid ghg^{-1}=h, \forall h \in H\}$. Therefore, $\psi$ is an embedding of $G$ into $\operatorname{Sym}(H)$ if and only if:
$$g \in G \setminus \{e\} \Rightarrow \exists h \in H \mid gh \ne hg \tag 1$$
Having said that, I can't conclude whether $(1)$ is false for every such pair $(G,H)$, or instead it does hold for some.
Edit (based on the accepted answer)
$$\operatorname{ker}(\psi)=\{g \in G \mid ghg^{-1}=h, \forall h \in H\}=C_G(H)$$
so that: $(H \lhd G) \wedge (C_G(H)=\{e\}) \Rightarrow G \hookrightarrow S_{|H|}$.