Organ Pipe Cactus National Monument in south central Arizona is an absolute gem, and worthy of much more exploration. A declared UNESCO biosphere reserve, the park protects over 500 square miles of Sonoran Desert, including the somewhat rare senita and organ pipe cacti. The Ajo Mountain and Puerto Blanco drives are popular (and easy) ways to fully enjoy the beauty of this desert environment. Other scenic drives are available to high-clearance vehicles and mountain bikes (another fun way to see the park). Many hiking trails will get you out into desert itself, the most popular (with me) being Arch Canyon, Bull Pasture, Estes Canyon, Lost Cabin, and Red Tanks Tinaja — but precautions must be taken for high temperatures and desert critters. Another great hike along an old wash will take you to a hillside populated with hundreds of petroglyphs (and past an old, deserted homestead), but you will need to do a little research (or ask a ranger) because this one is not advertised (likely because of the petroglyphs). A fall or winter visit here will allow you to take in everything that this monument has to offer.
Some pics (click to enlarge)