Use a temperature sensor to measure the ambient temperature (in Celsius) once every 30 seconds and post it to a ThingSpeak Channel.

For example, if I let my ESP8266 and temperature sensor collect data for a few minutes, this is a log of the ambient temperature in my room:

IMPORTANT! If you are using real hardware, the ESP8266 boards can only detect 0 - 1.0 V on the ADC pin. This means that you'll need to use a voltage divider to limit the range of the output of the temperature sensor to 1.0 V. To learn more about the ADC and other pins, see the Adafruit Feather HUZZAH and SparkFun ESP8266 Thing Dev Board pages. If you do not use a voltage divider, you could damage your ESP8266!

Hint: String()  can be called like a function to create a String object out of a float. If you specific String(some_float, 2)  it will return a String version of the number that has been rounded to 2 decimal places. You can read more about String() on the Arduino reference page.