Two liquids that react to produce a gas less dense than air at room temperature
If the other products aren't significant, I'll recommend the all famous alkali metals' reaction with water:
$$\ce{2Li(s) + 2H2O(l) -&gt; 2LiOH(aq) + H2(g)}$$
Hydrogen gas is less dense than air, both at RTP and at STP, because the main components of air (by mole percent) are $\ce{N2}~(78\%)$, $\ce{O2}~(20.95\%)$ and $\ce{Ar}~(0.934\%)$ - all of which have a molar mass larger than hydrogen.