Nature Travel in Southern Arizona

The observatories outside Tucson. The drive is a bit thrilling, but the mountain top is a "Sky Island" There are animals and plants there that are found nowhere else.

KOFA mountain range, home to the only species of Palm Tree native to the state of Arizona. Organ Pipe National Monument. Only place Organ Pipe Cactus grow. Be certain to visit the springs. Purely natural, as it has been for centuries, well except for the trail.
I would also recommend the Tohono O'Odam reservation and Casa Grande. San Xavier Mission, is a worthwhile look at 18th century Arizona history, and you should go past it anyway to Highway 82 east from Nogales and into the Southeastern terrain. North of Tucson, at the Sacaton Exit from I 10 there is a Native American museum with an outdoor display of an ancient village, they also have a restaurant, I think you can find some great Mexican food in Tucson, but I am not a local there and don't want to lead you astray.

For adventure travel, a day on Growler Wash between Organpipe and Ajo is a genuine adventure. CAUTION. When you get to the north end of it you need to find a road that goes east, or you will end up at I ten...and it is a long way. Also don't do it in the wet season. Check the weather. It can be done in any high clearance vehicle, though it is quite isolated.
Painted Rock Dam is not natural, but there is a really fascinating canyon on Centenial Wash, There is an improved road from just South of Arlington through the canyon to Gila Bend. You Might need the gates opened at the Arlington Feed company to get on the road. A great, but small riparian habitat is on Old highway 85, Arlington again, toward Gial Bend, the dam on the Gila was dynamited during a flood, years ago, Wher the bridge crosses the river. The Fact is that you will find a lot of undisturbed habitat on the Gila between Buckeye and all the way to Yuma.

Arivaipa Canyon, is a preserve, this is a link to the Nature conservancy. http://www.nature.org/wherewework/northamerica/states/arizona/preserves/art1946.html
In the central part of the state you might want to see the Boyce Thompson Arboretum, It is on Highway 60 west of Superior.
From Tucson take Florence highway to Highway 60 and then go east. Beautiful spot, loads of native and foreign desert plants.

I will think of twenty more as soon as I post this.

Karl: Thanks for the suggestions. Some are a little out of the range that I am focusing on for now, but I definitely going to keep them in mind for later when I expand to further out.

Amelie: I do not know what you are talking about archaeological site, but I doubt it was actually settled by Romans. Also, it would not really fit into my nature theme of my guide book.