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)