documentclass{book}



setcounter{tocdepth}{2}

begin{document}

tableofcontents

addtocontents{toc}{protectsetcounter{tocdepth}{2}}

part{One}
chapter{Foo}

section{Foo}

subsection{Foo stuff}



addtocontents{toc}{protectsetcounter{tocdepth}{1}}
setcounter{tocdepth}{1}

part{Two}
chapter{Foo}

section{Foo}

subsection{Foo stuff}



end{document}