For even more fun, turn your notepad into an interactive JavaScript environment:

  data:text/html, <html contenteditable onkeyup="eval(this.innerText)">

Then you can upgrade it by pasting in something like this:

  this.onkeyup = function () {
    this.style.backgroundColor = 'white';
    try {
      eval(this.innerText);
    } catch (e) {
     this.style.backgroundColor = '#FFAAAA';
    } 
  }