The Time Travelers (1964)

Starring Preston Foster, Philip Carey, Merry Anders, John Hoyt, Dennis Patrick
Directed by Ib Melchior
(actor & director credits courtesy IMDB.com)

A trio of scientists succeed in opening a portal 100 years into the future, where Earth has been decimated by atomic war, and the human survivors must fight off attacks from fearsome mutants.

One would not normally describe a movie about a post-apocalyptic Earth as "fun," but this film certainly is, plunging the three scientists and a lab worker into a future where after a narrow escape from the bloodthirsty mutants, they are indoctrinated into the survivor's camp, where they marvel at scientific advances, and the filmmakers add in humor and romance, abetted by a light-hearted musical score by Richard LaSalle.  Director Melchior, who worked on several other sci-fi pictures in the 1960s, collaborated on the script with David Hewitt, who also provided the film's special effects, which are pretty low-tech, even for 1964.  The portal into the future is obviously a doorway onto another set which the actors simply step through, and a matter transference device seen later looks to be recycled from a magician's disappearing act, but overall the effects are part of the film's charm, and I had no problem with them.