How can I return a random value from an array?

You can store the messages array and calculate the message to show as you go, like this:

var messages = ["Good!", "Great!", "Awesome!", "Super!", "Nice!"];
function getMessage() {
   return messages[Math.floor(Math.random() * messages.length)];
}

Give it a try here, then just call getMessage in your .text() call, like this:

label.addClass("valid").text(getMessage());