Hiding and showing elements when you tick a checkbox
You can use the checkox's "onclick" event to call a function. This event will fire even if the user uses the spacebar to tick/untick the box.
I've made an example of hiding a drop-down box and showing a textbox in its place when you click a checkbox. You can see it here:
In theory it would be the onchange event, but some browsers don't fire this until the checkbox loses focus, so it wouldn't happen as soon as you check or uncheck it.