Develop slow smoke for mini windtunnel
The DLR (German Aerospace Center) has developed a couple of DIY instructions for school education. Among these is a PDF in German for the construction of a wind tunnel.

For the smoke, they suggest to use either 


incense (the sticks or cones that you usually find in asia shops or esoteric shops) or a   
fog machine, like the ones used for entertainment (dance clubs, etc.)


In the latter case, a mixture of 1,2-propanediol and water is heated and pressed through a nozzle at elevated temperature. Given that properly insulated machines, build for safe use at home, are available from DIY electronics shops for roughly 30 € up, I wouldn't start to build one myself.