How to produce fumes in chemistry lab?
Aqueous ammonia and hydrochloric acid are solutions of gases. 

In a fume hood place two beakers with aliquots of these two solutions (one in each beaker, don't mix) side to side. 

Did I mention the fume hood?

You'll see the formation of solid ammonium chloride from the reaction of the two gases.

$$\ce{HCl(g) + NH3(g) -&gt; NH4Cl(s)}$$

Don't forget to clean up the mess!