So many things!
We always do a "greatest hits" day on our last day and while it changes from trip to trip, it probably sums up what is most essential.
From my December trip journal, on our last day we hit:
HS at rope drop - TSM twice, drawing class, RnR twice (DH only), cupcakes at Starring Rolls
MK - Pirates, HM twice, BTMRR twice, Dole Whips, Buzz, Under the Sea, TTA twice. BOG was closed for a media event or we would have had lunch there.
Epcot - Stroll through WS for night pictures, Livin with the Land, Kaki Gori, Boulangerie Patisserie, Maelstrom, soft pretzel in Germany and dark chocolate caramels for later.
AoA food court - Caprese sandwiches
Lots of other essentials didn't make the last day list but I would be sad if the trip did not include them - Boma, It's Tough to Be a Bug, Kilimanjaro Safari, Muppets, Tiki Room, Soarin', MK cinnamon rolls