1. TheIndieStation beat me to it, but I was going to say add some movement. You could have the background scroll, and not only could you have the clouds move into view, but you could have them move with a parallax effect as the mouse moves around the screen.
2. Limit the number of fonts you use. It looks like you have three. It would look better if you use one font, but change it for different uses. E.g. use different colours and sizes, but try not to have too many variations if you don't need them.
3. Buttons to enable/disable music and sound effects would be useful.