Best use for an old smartphone?

zaba writes: Ask Slashdot:
The original iPhone was a dream come true for me. Phone, camera, mp3 player and data all in one device. It had more cpu and memory than my first computer!

Several generations of smartphones later, my wife and I have some random smartphones (some iPhone, some Android) lying around. Between privacy concerns, bad batteries, etc. these phones are not worthy of donation.

So, I ask you, Slashdot readers, have you done anything fun with an old smartphone? Any suggestions/ideas?