I can pretty safely call this the birth of the "light" Star Trek episode. Even though previous episodes may have featured concepts or other elements that were a bit silly, they were there to further the drama or advance character. It was the execution that made them legitimate.
Here is an episode that has none of that. It doesn't quite present itself as an issue, however, because it never attempts to be anything more. No signature Trek ambition makes for an odd but enjoyable excursion as the Enterprise travels to an amusement park planet that reads the crew's thoughts with a device that looks like a cheap TV antennae, then quickly produces whatever it is they are thinking of.
The tone of the show is set in the opening teaser: tired and sore Kirk is complaining to Spock of back pains. As his yeoman begins to massage his back, he assumes it is Spock, to no protest. I find it amusing that he is okay with the on-duty massage initially, but orders it stopped when he finds out it wasn't Spock. "Dig it in there, Mr. Sp--"
This silliness embodies the entire episode, which plays more like a classic Shakespearean comedy in the vein of "Midsummer Night's Dream." It even has what is now considered a storytelling cop-out: the deus ex machina. After the crew encounters all sorts of crazy shenanigans with The White Rabbit, old lovers, a samurai, a tiger, and Don Juan, they are saved by the appearance of a caretaker who makes everything right. Actually, the appearance of this character makes the episode itself pretty inconsequential. This minimizes any offense viewers might have taken by its flightiness.
Another thing that amused me, unintentionally, was that even the facts we learn about some of the character's pasts through their fantasy do absolutely nothing to further their character! We meet an old lover of Kirk's, but this relationship not only goes nowhere, it gives us no more understanding of him than we had before. This becomes funny when McCoy's budding relationship with Yeoman Barrows teeters on genuine feeling ... an interaction which is afforded only a fraction of the Kirk/Ruth screen time!
There's also a whole slew of funny interactions, both on the planet and off. Such as Spock describing an irritable crewman who refuses shore leave. Kirk demands that his duty take precedent, and that the crewman relax. "What's his name?" To which Spock replies, "James Kirk." This sets up a quote from Kirk that caps off the episode:
"The more complex the mind, the greater the need for the simplicity of play."
In a brief (but appropriate for Shore Leave) drop of my analytical guard, I will admit to this being one of my three favorite episodes as a child. No surprise, right?