It has nothing to do with what is on the post or what can be seen. Highten levels of security since 9/11 have put the MPs and civilian guards on alert to people standing near the fence lines. They are not going to run you down but you do stand a good chance of being talked to.
That being said, the majority of the installations in the US are open to the public. Just stop at the gate (have your license, insurance, and vehicle registration) get a vehicle pass. Several parks and nature trails on Fort Huachuca. Even a cache in Garden Canyon if you are coming down this way.
Oh, be prepared for people to be looking under your hood, trunk, under the car, in the car and maybe a dog if you hit the gate at the wrong time.