I do not care that much for the DC zoo. It is rather small and most of the animals are in cages instead of big outdoor enclosures. IIRC it is also a bit off the beaten path in terms of the other attractions.
I'm not sure when you went, but most animals are in outdoor enclosures, as opposed to cages. (And, of course, there are the pandas!) It IS pretty hilly, though, and if you take Metro, I recommend getting off at the Cleveland Park stop instead of Woodley Park/Adams Morgan/Zoo--the walk is flat from Cleveland Park, but uphill from the Zoo stop.
Natural History has a lot of kid-friendly exhibits, and the monuments and memorials are free.