The night we went was very wet and very cold (exacerbated by the rain), but we still had fun. The first thing we did was ride the train. The kids love any kind of train, and it's a nice overview of most the lights.
This year, they've started offering these great ZooLights hot cocoa cups.
With the rain and the wind and the chilly temps, we opted not to stay as long as we normally do. We still walked around and saw all the lights, but there was no lingering the way we have in the past. I recommend checking it out. You have through the first of the new year. And definitely bundle up and prepare for rain (it is Oregon, after all)!
For more information: oregonzoo.org/visit/zoolights