Here's one possibility: I used the background package to place a Return button on every page of the document; the button is placed centered at the bottom of each page (the position of the button can be freely and easily changed). The button is a hyperlink to the ToC; this was created using the \hyperlink, \hypertarget mechanism provided by hyperref. For the "Return" button symbol I used \Return from the keystroke package, but you can use the symbol that better suits your needs:
\documentclass{article}
\usepackage[a6paper]{geometry}% just for the example
\usepackage{background}
\usepackage{etoolbox}
\usepackage{keystroke}
\usepackage{lipsum}% just to generate text for the example
\usepackage[colorlinks=true,linkcolor=cyan]{hyperref}
\makeatletter
\patchcmd{\tableofcontents}{\@starttoc{toc}}{\hypertarget{totoc}{}\@starttoc{toc}}{}{}
\makeatother
\backgroundsetup{
scale=1,
angle=0,
color=black,
position=current page.south,
vshift=20pt,
contents={
\tikz[remember picture,overlay]
\node[inner sep=0pt] {\hyperlink{totoc}{\Return}};
}
}
\begin{document}
\tableofcontents
\section{First section}
\lipsum[4]\lipsum[4]
\lipsum[4]\lipsum[4]
\section{Second section}
\lipsum[4]\lipsum[4]
\lipsum[4]\lipsum[4]
\end{document}
An image of the resulting document:

A zoom of one of the pages, showing the button:

In case you have an older version of background, you can use the old syntax (this will also work in the newer versions):
\documentclass{article}
\usepackage[a6paper]{geometry}% just for the example
\usepackage{background}
\usepackage{etoolbox}
\usepackage{keystroke}
\usepackage{lipsum}% just to generate text for the example
\usepackage[colorlinks=true,linkcolor=cyan]{hyperref}
\makeatletter
\patchcmd{\tableofcontents}{\@starttoc{toc}}{\hypertarget{totoc}{}\@starttoc{toc}}{}{}
\makeatother
\SetBgScale{1}
\SetBgAngle{0}
\SetBgColor{black}
\SetBgPosition{current page.south}
\SetBgVshift{20pt}
\SetBgContents{\tikz[remember picture,overlay]
\node[inner sep=0pt] {\hyperlink{totoc}{\Return}};}
\begin{document}
\tableofcontents
\section{First section}
\lipsum[4]\lipsum[4]
\lipsum[4]\lipsum[4]
\section{Second section}
\lipsum[4]\lipsum[4]
\lipsum[4]\lipsum[4]
\end{document}