Yes, there have been discussions on this subject before, and probably better analysis than what I'm about to deliver... but since I'm too lazy to dig for said superior analysis, I might as well just state my own opinion of it.

I'd say the only thing one can be certain of about it is that yes, it is more than just gymnastics and an open shirt. However, it's not really made 100% clear exactly what the person in the backseat of the car actually sees. In my opinion, the only clear indicator is the glimpse we get of Miki's vision, which begins prematurely as Kozue inadvertently(?) blindfolds him. He pictures himself driving the car, with Anthy at his side. This is basically what Miki believes he wants most, the solution to all his troubles.

Based on that, I can only speculate that what the other duelists see are also personally tailored visions depicting their hearts' innermost desires. Saionji claims to have seen "it", "The world"... but doesn't specify anything else. After all, he also states that Utena nor Miki would understand his explanation, such was his revelation. Although in Saionji's case, I get the impression his own subconscious blurred the specifics of it in order to keep him from losing his grip on "reality", as it were...

Well, this is just what I managed to come up with anyway. Someone else probably has it down more concisely than I do, but whatever...