Hidden Valley was really hidden with the only way in via the long gulch on the north side through which water drains from the valley. In the beginning of 20th century a local rancher William Keys blasted the walls on the south side of the valley to let his cattle graze there.
Now one of the nature trails in Joshua Tree National Park makes a lasso loop through the valley with views to the variously shaped rock formations and interpretive signs talking about Mojave Desert flora.
Off-trail hiking and rock climbing are popular in Hidden Valley, although Park administration discourages these activities due to their high volume and consecutive significant negative impact.
Check out the Official Route and Triplogs.
WARNING! Hiking and outdoor related sports can be dangerous. Be responsible and prepare for the trip. Study the area you are entering and plan accordingly. Dress for the current and unexpected weather changes. Take plenty of water. Never go alone. Make an itinerary with your plan(s), route(s), destination(s) and expected return time. Give your itinerary to trusted family and/or friends.