Generate heat on an android phone (from code, of course)

Basically turn on everything you can that uses power, in addition to the CPU:

• Keep the screen on and bright (if you have an OLED fill it with white). The best way to do this is with an activity setting FLAG_KEEP_SCREEN_ON and maximum brightness.
• Monitor location with GPS.
• Send and receive data over the network.
• Use OpenGL to draw some moving graphics (to run the GPU, and get the surface compositor going).
• Play audio.

Something you can't do in software but is also a big battery drain is being in a low signal area, so the cell radio needs to bump up its power.

Or just run Google Navigation... it already does nearly all of that, and is indeed and big battery drainer and device warmer. ;)

I would try to also write data to SD-Card and phone storage at a high frequency and enable the Wifi and GPS sensors. Also set the screen to the maximal brightness and maybe show some complex graphics or videos on it too.

What works for me is to connect it to USB to charge up and run Wifi Tether. That gets the CPU up to about 43 degrees Celsius at least.