The weather should be good this time!
Zoo Lights is free. The Small Mammal House, Great Ape House, Reptile Discovery Center, Think Tank, and Kids' Farm will be open with special demonstrations and talks from interpreters and zookeepers. If any Africa Trail animals are out I'd be happy to put on my (metaphorical) interpreter hat for questions, answers, and explanations about what those critters are doing and why.
Tubing, Train Rides, and Conservation Carousel all have fees.
MetroRail: Take the Red Line to Cleveland Park (it's an easier walk than from Woodley) and walk south on Connecticut Ave. The Zoo entrance is about a mile, on the east side of Connecticut Ave. There will probably be weekend track work so plan accordingly.
MetroBus: Lines L1, L2, and L4 stop at the Zoo's Connecticut Avenue entrance. You can also check wmata.com for delays and road & track work updates.