An digital temperature sensor for your phone. Thermodo is a good one compatible with Android and iPhones. However, you MUST get the 6 inch (20 cm) audio extension cord as well since the phone will heat up as you use the provided tracking app.

I work as a mechanical engineer, so knowing the temperature of the air around me is useful and interesting. At work, I have better equipment that gets much more accurate, but this device (with extension) gets me within +/- 2.5 degrees. I believe the new Samsung Galaxy phones have this built in, but I haven't tried them yet.