Darwin day: visit tidepools, forests, etc and poke around to find interesting animals - or an aquarium with a touching pool, something like that. At the end of the day, a mulit-taxon gumbo with various veggies and meat from multiple groups: shrimp, fish, chicken, maybe some calamari... I don't usually eat a lot of meat, but I can make an exception for religious reasons
Carl Sagan day: stay up all night, especially going out to the country somewhere, to look at the stars? Maybe visit an observatory if one is near you? Other suggestions?
Darwin day: I like your suggestions. A good day for a trip to the zoo.
Carl Sagan day: Great suggestions there too. I would add replaying "Cosmos" every year on his birthday, like certain films they show annually around xmas. Maybe check out images from the Hubble telescope on the web.