I tihnk Helium is a good one - on top of the balloon idea, it was an element first discovered by studing Fraunhofer lines in the heliospectrum, marking an important step in that sort of chemistry. Links with physics too, since a Helium Nucleus is an alpha particle (radioactivity), and Spectral lines relate to photon emission and electron orbit quantisation - the Old Quantum Theory... lots of possibilities.
You could, too, bring in Nuclear fusion, which involves Deuterium and Tritium to make Helium and hopefully release lots of energy (and neatly bringing in Mxy's idea of Deuterium into the bargain), and of course the little squeaky-voice effect of helium/ balloons...
Supercooled helium at 3K or so reveals some interesting behaviours of low-temperature matter...
Basically go for helium Mith. That's my advice.