In my opinion going to the Botanical Garden is a must if you're in Vegas during the holiday season. It's free. It's tough to turn down something that's free in Vegas.
I'm always impressed by the amount of detail they put into it. The flowers, Christmas tree, polar bear, etc. are all very nice to look at.
I recommend taking pictures and even some video if you can. It gets packed there so you have to be patient. A lot of tourists visit this spot. This particular display is up from early December through early January.