The vast majority of 1967's Star Trek episode number 19 called "Arena" was shot at the Vasquez Rocks county park, which is located about 30 miles north of Los Angeles. With the exception of new vegetation, the park itself has changed very little in the thirty three years between when this episode was filmed and when I visited it in May 1999. In fact, all of the parks landmarks are still there, and many of these landmarks can be seen at various points in the episode.
I have taken the liberty to photograph many of the frames from "Arena" (sorry the quality isn't the greatest) and place them here for you to compare to the photo of the park taken in 1999. All of locations shown in these frames from the episode can be found in my photograph of the park.