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"I'm not talking about OpenGL and using the ndk but creating Android activities"

I'm not really sure what you mean. Any program for Android which is 100% C code is going to be using the NDK in most cases. Including ones creating an activity natively ( http://developer.android.com/reference/android/app/NativeAct... ).

Seemly Marmalade SDK create android apps without using NDK...

Seemly what it do is compile a valid ARM code using arm abi (not even the linux abi), and then there is a component that do use android OS stuff to load in the memory the code you compiled for arm abi...

Funky stuff. And patended. (Marmalade website make VERY CLEAR that their "Secret sauce" is their loaders, that are also very much patended...)

Well I suppose the Android equivalent would be writing raw Dalvik bytecode. Certainly possible, but probably not a lot of fun. It wouldn't be nearly as informative about the underlying architecture in any case.

I'm not sure how the JNI works on Android, but theoretically, yes. I am not a huge android fan however, so I probably won't be the one to attempt it.