I did it two years ago - great experience. There's two parts to it: theory, in the form of lectures covering taxonomy, husbandry and such, along with practical work with all of the sections in the zoo (invertebrates, herps, birds, small mammals etc.). You'll get to see a little bit behind the scenes and the staff/volunteers who'll be with you will do their best to answer anything you want to know. If you're ok with the fairly large cost (which does work out as significantly cheaper than the keeper for a day scheme), I'd fully recommend it.