Well, you can play Pandora in the background and still use Navigation. Or, vice-versa. If you want quick access to Pandora set a widget on the home screen and you'll be good to go
For other apps, you'll just have to switch back and forth. Remember that you can always press and hold the home button to bring up a quick menu of the recently used apps for quick access. This way you won't have to go into the app drawer and find the app.