<input type="checkbox" name="one" id="one_id" onClick="showSpoiler(this)" value="Show/Hide">
<div class="inner" style="display:none;" id="inner_id">
function showSpoiler(obj)
{
var chbx = document.getElementById('one_id');
var inner = document.getElementById('inner_id');
if (chbx.checked == true)
inner.style.display = "block";
else
inner.style.display = "none";
}