How can I do this? I have big problem with very cold winter while I'm waiting for a bus. I was searching for something, but everything I found is how to stop overheating. App with big size is not good on my phone. This can be something for rooted smartphone.
You're kidding, right?
But for the sake of argument: write a very inefficient OpenGL ES app (http://developer.android.com/guide/topics/graphics/opengl.html) this will put some strain on your CPU and GPU and the phone will get quite warm. You don't wont it to OVERheat, b/c this will damage your phone, so be cautious.
Btw: it will drain your battery fast.