Different approaches to Android plugins

I'm considering trying my hand at creating Android plugins for Unity but I'm a bit unclear about the different approaches.

I have read the manual page about this but it was confusing to my uninitiated mind.

I know that you can create a plugin by making an android project that extends the UnityMain activity and calling the java functions from C# (this seems to be how most people do their plugins). This doesn't seem ideal because you can only have one plugin that extends the main activity. What are the other approaches to doing this and are where should I start to learn them.

thanks!