Cairo Zoo is one of the oldest in Africa, established in 1891 on the grounds of a former Khedival botanical garden in Giza, directly across from Cairo University. At its founding it was considered one of the world's finest zoos; it remains large (over 80 hectares) and has an interesting collection, though standards of care have varied over the decades. The zoo is primarily patronised by Egyptian families and schoolchildren.
The grounds themselves are interesting independent of the animals: mature botanical garden trees, ponds, and the original 19th-century garden layout survive in the newer zoo infrastructure. The visit is most interesting for Egyptian social observation — it's a popular destination for family outings, with traditional food vendors and tea sellers throughout. The adjacent Orman Garden (adjacent but separately entered) is another good green space.