You can use innerHTML, or if you want to something more complex you could append a new element, for example:
HTML
collections
Javascript
function show() {
var hidden= document.getElementById("hidden"),
newElem = document.createElement("div");
newElem.innerHTML = "
collection 1
collection 2
etc...
";
newElem.style.color = "red";
hidden.appendChild(newElem);
}