The Boyce Thompson Arboretum is a huge open area filled with all sorts of plants from around the world. You'll find every sort of cactus here, along with deciduous trees, flowering plants and evergreens.
The Arboretum is far enough outside the city limits to have really clear clean air, and its distance from the city lights of Phoenix makes it the perfect place for stargazing. Many people take telescopes here regularly to look at special celestial events. If you are a novice, there are guides available to help you out.
It's well worth the drive, day or night.