A space to excavate, build and form is a great addition to any play space. Arrangements of rock, branches and various stuff, can look indiscriminant but in reality is the design of a complex wonderful imaginative experiment.
This type of play is the freest for the imagination there is. Play using collections of objects is really an exercise of the use of symbolism. Each piece becomes part of a greater idea. These ideas can be fluid and give a chance for a social order to be experimented with.
Consider this an important aspect of your play space. You do not need a large space to integrate this opportunity into a play environment.