A top mounted carrier board for the MQ-series sensors, much like the Pololu board, would be great for little data collection packages.

Source for sensors:
http://www.pololu.com/category/83/gas-sensors
https://www.futurlec.com/Gas_Sensors.shtml
http://www.electrodragon.com/product/mq-2-mq-3-mq-5-series-sensors-variables/ (Inexpensive!)

Of course, other sensors such as those found at Figaro, would be cool to have as well.