vintagegeekculture:

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.