The most lasting contribution that the Star Trek animated series gave Star Trek was the concept of the holodeck, first seen in the animated show.
There were plans to feature something like the holodeck in the original series, but there was simply no budget for this idea. The holodeck is one of those things, like emergency saucer separation, that people associate with the next generation, but the original series could do it too.
There’s a lot of discussion on how Star Trek was a team effort by a lot of different writers (notably Gene L. Coon, creator of Starfleet, the Klingons, and the Prime Directive), but the holodeck was purely Gene Roddenberry’s idea. In the series bible from 1966, he wrote that “entertainment in the Star Trek era is three dimensional and like a film, except it happens all around you and is interactive.”
I’m old enough, I watched these first run. I thought the ‘rec room’ was a logical, but awesome, thing to be in the 23rd century. And saucer separation was one of those things I always considered canon because of Franz Jospeh and Steven Cole. I can’t remember if they ever did in in TAS.
In “The Apple,” when the god-computer Vaal held the Enterprise in stasis and unable to leave orbit, Kirk and Scotty considered a risky plan: they wanted to separate the main section (the saucer) from the engineering section and the engines to escape. From the reactions and the context of the conversation, this was considered an incredibly risky plan of last resort, and the term “jettison” was used to describe this process…meaning that it is reasonable to assume that the original series Enterprise could perform a saucer separation maneuver, but it was an emergency escape move, and it was very unlikely it could reconnect afterward.
By the time we reach Star Trek: the Motion Picture, a big part of the film was going to be focused on an Emergency Saucer Separation at a dramatic moment. This sequence was scrapped, but Andrew Probert actually did concept art for it, and since Probert designed the movie Enterprise, he built lines and cracks for this maneuver into the design of the ship.