Your transportation options from the Grand Floridian back to the Animal Kingdom Lodge will actually depend on what time the Magic Kingdom is closing that evening. Transportation continues until about 2 hours after park closing.
If possible, take the monorail back to the Magic Kingdom and then transfer to the Animal Kingdom Lodge bus.
If the Magic Kingdom closes early (say 7 p.m.) then you’ll need to take a bus from the Grand Floridian to Downtown Disney. At Downtown Disney, you will transfer to your resort bus.
Go to the
calendar and see what the Magic Kingdom hours are for your visit. Then you’ll know which plan to use. (Your other option would be to take a bus to whatever theme park is open the latest and then transfer to the Animal Kingdom Lodge bus.)
Yes, a taxi would be the quickest.
I would also recommend walking around the Grand Floridian. If time allows, conclude your evening with a magical view of the Magic Kingdom's nightly fireworks. Walk to the boat dock and watch the Wishes fireworks display. Enjoy your dinner with Cinderella!