Seoul Zoo: The Full-Day Animal House, Korea
Because animals are animals and there isn't much else to say about many of them, I'll let the pictures speak for themselves, mostly.
There is a third, seven kilometer loop around the entire zoo that looks like it would make for a very nice 2.5 hour hike.
Take Subway Line 2 to Seoul Grand Park, and take Exit 2. You'll walk straight through the wide plaza in the middle of the car park, where vendors set up selling lots of interesting food and souvenirs. I bought some tasty donuts, but you could buy boiled beetles if you'd like. Once you reach the loop road beyond the car park, you can catch the Elephant Train shuttle to the zoo or just walk to the right, following the road over the lake.