Flammable lighter than air gases (safeish)
I would propose to use hydrogen, as it is pretty easy to obtain right there where you need it and it's the lightest gas ever. 
You can even watch a  video where the whole setup for the bubble experiment is described  (at the end). 
Besides hydrogen, methane looks good to me, and it is also in our everyday life, being the natural gas. But the setup for using household methane is imagined more complicated to me than the one with hydrogen.
Just be very careful with any flammable gas, don't make it another Hindenburg :)