The Serengeti is a huge park, full of wildebeest (I was there during the Great Migration) and zebras. Every now and then, a rock formation stands out against the landscape, sheltering groups of lions or a cheetah.
It takes several days to explore, and it can be a wonderful experience to sleep in a tented camp inside the park.
One word of caution: avoid the Great Migration. It's certainly a unique event, but unfortunately, it attracts too many tourists. I found myself watching the crossing of the Mara River along with at least 50 other cars. Only the closest five or six could see anything; everyone else could only see the roofs of their cars.