Students from Carnegie Mellon University have taken the idea of an LED floatie further with an air quality sensor. The DIY air-sniffing balloon project can be built fairly easily and on the cheap. All it takes is a bit of circuitry skill.
The air quality sensor is only $10, and the rest of the supplies can be found cheaply. Tri-colored LEDs change color depending on the data given by the sensor. Fitted with rechargeable batteries and a microcontroller, the LEDs glow green, yellow or red based on low, average, and high amounts of pollutants in the air. Check out how to build your own.