<div id="spoiler" style="display:none"> This is the content </div>
<button title="Click to show/hide" type="button" onclick="if(document.getElementById('spoiler') .style.display=='none')
{document.getElementById('spoiler')
.style.display=''}else{document.getElementById('spoiler')
.style.display='none'}">Open/Close
</button>
отсюда
гуглится на раз по "simple spoiler code"