Yes, totally Boo at the ZOO, fantastic Halloween event that beats the light out of any Halloween event for kids in the entire DC area. Tons of people and tons of candy stations everywhere and the decorations and lights. I don't remember the details but somehow having membership made it much less expensive and well worth having the membership for the reminder of the year because of that.
You should find more info on this:
https://nationalzoo.si.edu/events/boo-zoo
Also, yes once you have the membership parking is non-issue and you feel more motivated to go to the Zoo because you don't need to calculate so much extra time.