Not sure if you're talking about an actual ferris wheel, but since you mentioned renting a private unit, I assume you're referring to the pods on the High Roller observation wheel?
**First idea: if you want to get naughty w/o risk of arrest, why not use remote control vibrators on each other? With the distracting noise, music and lights in the pod, this will be your best bet.**
I have now been on the wheel over 10 times, both during daylight and nighttime. A friend of mine considered renting one for ourselves one night, not to be "naughty" but to just experience it together with no other distractions. With the exception of two rides, there were less than five people in the pod with us. Meaning: when I was in there with just one other person, we seemed to be hooked up with another "duo" or couple.
Regarding getting naughty...as others have said, yes, there are cameras. Not to mention the guy on the TV screens who talks you through the whole ride in between songs. The inside is cold/steel/not what I would consider sexy or sensual....maybe to those with a fetish for cold...but the people in the pod above and below can see all that you do as they are essentially floor to ceiling windows. When the pod above me is about to hit the "peak" of the ride, I've seen those passengers wave down into my pod, and we've even taken pictures of each other with our smart phones. All of this would likely be incredibly distracting and/or annoying.
Lastly, my most recent ride took place just before closing time at 1AM with some youngsters (I'd say 12-16 years of age?) in line with their parents.
All of this said, if you're talking about the High Roller, I would say maybe ride on it with your "special someone" and gently kiss or do a subtle grind but no way you can get away with doing quickies or swapping. Hell, it's only 30 mins anyway!
Think about the remote control vibrators...in fact, I think my next time up I'm going to try that myself!
Enjoy Vegas and I hope you can satisfy those bucket list wishes