There isn't any way to easily make Metro UI apps. Games are programmed with XNA (or OpenZDK, which is more difficult and isn't supported by Microsoft), which is designed for programming games. Programming apps is a little more difficult than it would be on other platforms because there is no standard UI. You have to make the UI from scratch. You can use XNA to make games for Windows as well (as well as Windows Phones and Xbox 360s), so you can try it out before you get a Zune.
Windows Phones can be programmed with both XNA and Silverlight. It is easy to make a Metro UI with Silverlight, but unfortunately it is not available for Zunes.