There were three more major projects that Kirby undertook at Marvel beside his return to Captain America.
First was an adaptation of the film 2001.
The series continued on past the end of the film and in it Kirby introduced one of his final lasting creations Machine Man.
I'm not certain what it is about this character but for some reason comic book creators are constantly using him. He turns up for a year or two and then vanishes for a short time and then comes right back again with someone else doing a completely different take on the character.
Second was a return to the same type of ground that Kirby covered in Kamandi at DC comics. The new series Devil Dinosaur was a forerunner to modern comics in some ways as it was created specifically to tempt licensors. No one was interested in the property and so it was canceled after nine issues.
While the stories are atrociously bad as Kirby's writing typically got worse and worse as the decade continued, I do have a special place in my heart for this series. It has some wonderful pages of art featuring freakish dinosaurs fighting other freakish dinosaurs. It's a situation where I'm almost tempted to break my book so you could see the crazy double page spreads that peppered the series.
Finally there is The Eternals which Kirby loosely based on the book The Chariots of the Gods which was popular at the time. That book popularized the theory that aliens were behind most of the world's mythologies and assisted in building many of its monuments. This book had the most traditional look of Kirby's last Marvel creations and any of it would fit in fine with his efforts on Thor or the Fantastic Four in the 1960's. It also is the best written of his work since New Gods but that isn't saying much. Kirby's big concepts are on full display in this series that I consider to be a last hurrah for him. It ended after just nineteen issues.
Toward the end of 1977 Jack Kirby left Marvel again this time never to return. It was not the end of his work on Marvel characters, however. He immediately went to work with design on the Fantastic Four cartoon. Yes, H.E.R.B.I.E. the robot replacement for the Human Torch in the late seventies series is a Kirby creation. He would remain working on various animated projects (most notably Thundarr the Barbarian but there were others) until his retirement.
He only did one more thing in comics in the 1970's. Starting in late 1979 Jack Kirby did an adaptation of the Disney film The Black Hole for newspapers:
Kirby would eventually do a handful of comic additional projects in the 1980's and continue to provide an occasional pin-up or cover until his death in 1994 but his career in comic books that started in the late 30's was over.