What aspects of biology am I most likely to observe in a zoo?

The animals... for sure
Observe them... their features... their behavior... their artificial environment... their comfortability in that environment... there r lot many things ,you just need an eye and mind to observe all tah tn write a report... but for tha tfirst of all what u need is interest ,if u don't have it ,u won't work in a better way.

Enjoy.