Young animals are always a good bet - the cute factor... Look out for feeding behaviour (especially pigs as they can be quite amusing when being fed). Look for animals in interesting poses - horses looking out of stable doors, pigs reared up on sty walls and the such like. Sleeping animals are another good bet. A few portraits are worth a shot or two.
Basically keep your eyes open and shoot whatever looks good to you - and that includes what other visitors are doing and how they react to the animals...