APK Editor is a powerful tool that can edit/hack apk files to do lots of things for fun.
It can help us to do things like string localization background image replacement layout re-architecting and even ad eliminating permission removing etc. What it can do depends on how you use it. However to use it well we need a little bit professional skills. Don't be afraid some examples are given in the help page.
(Note: Manifest editing is not supported in free version)