Last time most of the original casts of both shows performed their characters.
Pebbles and Bamm-Bamm don't appear in the movie. It's possible that the point in time where the Jetsons traveled to Pebbles and Bamm-Bamm either haven't been born yet or have married and live on their own (i.e. I Yabba-Dabba Do! (1993) and Hollyrock-a-Bye Baby (1993))
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.
In the Flintstones episode "The Long, Long, Long Weekend", The Great Gazoo sent the Flintstones and Rubbles into the future that looks similar to the Jetsons world, but only Fred and Barney remember it when they went back to the Stone Age in Bedrock.
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).