Some capacitor experimentation could be very interesting!
Run a series of comparisons between rated voltage and failure voltages (different brands?).
See how the capacitor characteristics change over time the more they spark over (ESR, capacitance, dissipation factor, phase angle).
See if they consistently spark over at the same spot or if it continues to burn holes in the dielectric.
Explain why electrolytic capacitors are polarized and why they don't like having reverse polarity applied to them. Would also be fun to see if you could reverse the polarity of a capacitor (have it powered reverse-current limited for a long time).
And if you got some precise micrometers (or calipers), examine the dielectric breakdown voltage of some substances and compare with table values. (PTFE, air, PE, HDPE, 'vacuum', etc.)
(Just some ideas)