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))
The Jetsons and the Flintstones would later be a part of the Universal Studios attraction The Funtastic World of Hanna-Barbera (1990); both in the short film and as walk-around characters.
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).
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.