I agree that this area has lots of potential. However, this is only half of their outdoor space, there is a very large climbing structure to the right of this image. When this enclosure was built it did contain many shrubs and 2 or 3 tall trees, but these were quickly destroyed by the chimps. During the summer a large portion of the grass area is left to grow substantially long. The chimps are also regularly provided with enrichment to keep them occupied.
Its a huge area but the best enrichment they could have is some male company....but it seems they are still too scared of escapes because of weak fencing, and male Chimps on the rampage, so it doesn't happen.