Yokohama Zoorashia is one of the best Zoo in Kanto area. Famous for the animal called Okapii (pic above), this zoo has African theme. Zoo is quite far from central area of Yokohama but there are lots of parking spaces so I recommend going there by car. What I really liked about this Zoo was that animals are not in Cages and if they are in cages they are in pretty big one. It almost looks like a park.
If you are looking for a nice day out with your partner, I recommend this Zoo for walking around. There are restaurants in the park as well as there are benches and chairs so you could bring your own lunch.