I love Joshua Tree. There are so many hikes and views and rocks. It's a gigantic playground that I visit frequently. The Lost Horse Mine Trail is off Keys View Road and takes you to a historical mine that was in use from 1894 through 1931. Not growing up in California, this was virtually my first run in with gold rush history.
The hike is 4 miles if you just turn around at the mine, but you can extend the hike by doing the entire Lost Horse Loop, making it a little over 6.5 miles. Oh, and if you hike the Lost Horse Mile Trail, try not to walk on top of baby rattlesnakes like I did.