Last time most of the original casts of both shows performed their characters.
Pebbles and Bamm-Bamm don't appear in the movie.
When the Jetsons and Flintstones arrive at the picnic an instrumental version of 'Rock Around the Galaxy' can be heard. The song would later be heard again in Rockin' with Judy Jetson (1988).
It's well known by now that the Hooneymooners were the real life prototypes of the Flinstones; and The Dick Van Dyke show was the prototype for The Jetsons.