This is actually a pretty good list. Usually articles like that tend to not have much in the quality department.
Red Rock Canyon is such an easy day trip from Vegas (and a good excuse to visit the beautiful Red Rock Resort). Once you've done that, Valley of Fire is a good natural progression for a future trip as it's Red Rock on steroids.
We enjoyed the Neon Museum, but we went on a 110° day quick was a bit much. We want to check out their Brilliant show at night still.
Yes, you remembered correctly about the Ethel M cactus garden at Christmas time. I highly recommend it -- we've gone three different times now and it's been excellent every time. I'm not so sure that it's a must-do at other times of the year, although it's something non-gambling if you're looking for that.
One more I would add to the list is the Mob Museum. We checked that out for the first time on our last trip and it was excellent! If you haven't been, it's well worth the visit.
