Some of the hardest goodbyes were to the creatures who didn't even realize I wouldn't be back the next day... I've worked with primates (mainly gorillas) for the last three years, investing most of my week time in their health, training, entertainment, comfort, etc. What's difficult about leaving them for the last time is that they're not "contact-able" like our friends and family.
I definitely cried at the end of my last zoo day... but not before decorating the gorilla cages with my goodbye messages in chalk!
A funny thing Ramses did that afternoon on my last day: He was pulling his tongue out of his mouth and messing with it. I was like, "What's wrong, Rams?" He promptly came to the side of the cage where I was standing and stuck his tongue out of the bars to show me his "owie". He had an inflamed taste bud in the middle of his tongue. I felt bad that I couldn't "fix" it for him like he wanted, but I was flattered that he thought I might be able to. :)