I've also noticed massive savannah exhibits gain stars quickly. In every scenario I play, I always have a giant savannah exhibit, and try to include the following:
Thomson's Gazelles, Masai Giraffes, Reticulated Giraffes, African Buffalo, Gemsbok, Gerenuk, Black Rhinoceroses, Gerenuk, African spurred tortoises, giant sable antelope, common zebras, African elephants, Asian elephants.
There's a lot more I'm sure you can put in, but that's all I can think of.