Thermodynamic data are very easy to mine at the NIST webbook and fairly easy to interpret even for general chemistry level. For instance, combustion enthalpies or phase transition points (boiling point) of hydrocarbons as a function of chain length and branching points, or alcohols as a function of chain length and branching point, etc. You might, as an example, plot combustion enthalpies out for various alkanes, and then also plot on a per-carbon basis, where you can so the terminus effects disappear as the alkane gets larger.
Alternatively you can look at solution enthalpies and entropies for various ions dissolved in water to see what trends emerge as you go down and across the periodic table.
And so on.
There are plenty of spectroscopic data as well, but broad trends may be more difficult to make sense of at the high school level.
The NIST webbook is very reliable for data mining. Even better would be if you can get your hands on a CRC handbook.