The only thing you can really do it play with the lock screen and home screen wallpaper. So, have a look at the app "Pimp My Screen." It's got pretty good overlays so you can jazz up your lock screen (and add shelves and such to your homescreen). There are more apps that can help you create nice wall papers - this is just the one I use...
Unfortunately, that's all you can do with an un-jailbroken device.
Marilyn