There are lots of reasons to come here, which you'll learn as you peruse the other reviews on here, but my favorite 3 are mountain biking, hiking, and outdoor gyms.
1) Mountain Biking: This place has GREAT beginner mountain bike trails; perfect for getting yourself initiated into the sport of off-road biking or introducing your newbie friends into the sport. Park at the West Butte Parking Lot (at the light where you turn East into the Zoo parking lot from Galvin Pkwy, turn West instead and you're there). The mountain biking trails run from this point Northward along the West side of Galvin Pkwy and the South side of McDowell Rd. From these trails you can access a really cool outdoor amphitheater and watch the Oakland A's practice their craft.
2) Hiking: Awesome rock formations to hike. See other reviews.
3) Outdoor Gym: I am a HUGE fan of outdoor gyms because they provide a powerful full-body workout rivaling any commercial weightlifting gym for FREE. I happily found this outdoor gym quite by accident, and don't really see it advertised as a feature of this awesome park. it runs along the West side of the golf course. Park at the West Butte Parking Lot (See #1 for location).