Each time I visit Zion NP, I drive up Smithsonian Butte Road. It's a short 4wd road south of Rockville, UT that heads up to the top of the mesa and continues south to highway 59. I like driving this road because it's a nice break away from the crowds, the temperature is cooler on top the mesa, and the 4wd conditions are sometimes kinda rugged.
The hard part is the shelf road that goes up the mesa. It's short: less than a half mile long. Sometimes it's easy and smooth. Sometimes it's very hard and I have to stop and choose my line over/around big embedded rocks. I've never had to get out and scout the line or move any boulders.
On my last visit, I noticed that shiny new high-visibility warning signs had been posted at the top and bottom of the hill. I wonder what precipitated those. I didn't see any signs of anybody having fallen off the road.
If you have a 4wd car with high clearance (important in this case) then you might enjoy this road. On top of the mesa, there are more 4wd trails off the main road. They're also interesting, but more brushy and so you might get scratches. It's BLM land so you can camp in the area - at least 0.5 mile from the main road. There are plenty of "previously disturbed" camp sites along the side trails. I've camped there in the past when the Zion NP South Campground was full.