Cacti. Lots of it.
But it's more interesting than it sounds! I would ONLY come here on a nice day around sunset time. Come maybe half an hour before the sun really starts setting and then be amazed as beautiful shadows are cast throughout the garden (I'm not from the area, so I'm blown away everyday by a Phoenix sunset).
Also, do try to catch the Chihuly exhibit before it ends. I didn't find it as amazing as some, but it's definitely an interesting concept that is also greatly enhanced by sunsets ;-). And the DJ and live performers they get seem to be top notch, so the evening socials definitely look interesting if you're into that cocktail party sort of vibe.
Don't forget to bring your camera!