Personally, doing research in zoos, the biggest enrichment impact I can think of is forage feeding accmopanied by natural bedding (e.g., mulch). Primate have to spend a much larger portion of time searching for food producing activity budgets (for foraging) that are more in-line with what is seen in the wild. Other naturalist manipulanda include browse (promotes more appropriate nest-building/resting behaviors, also feeding/foraging), probe-mounds (for chimps, promote tool-use feeding).

There's a large literature for this field. You may also benefit from looking at the biomedical enrichment literature (promoting species-typical behavior). Many of the devices/enrichments are not naturalistic but have been associated with more species-typical behavior profiles & activity budgets after being introduced.

Another issue may not be the nature of the enrichment but the practicalities or its presentation (e.g., how presented, predictability, accessibility).

Just a few (recent)

Birkett, L. P., & Newton-Fisher, N. E. (2011). How abnormal is the behaviour of captive, zoo-living chimpanzees?. PLoS One, 6(6), e20101.

Mas, J., Stafford, R., & Jensvold, M. L. (2013). Captive chimpanzee preference for environmental enrichment: naturalistic vs. artificial.

Zaragoza, F., Ibáñez, M., Mas, B., Laiglesia, S., & Anzola, B. (2011). Influence of environmental enrichment in captive chimpanzees (Pan troglodytes spp.) and gorillas (gorilla gorilla gorilla): behavior and faecal cortisol levels. Revista Científica, 21(5), 447-456.

Yes, the focus tends to be on feeding enrichments since the species tends to be so destructive that options for other enrichments such as playthings are limited except perhaps for the young. Introduction of novel dietary items can encourage their instinct to trial foods.

Have a look at this article by Clark and Smith (2013). It's an interesting enrichment device that didn't necessarily involve food.

Article Effect of a Cognitive Challenge Device Containing Food and N...

This may be a helpful link: http://zoonewsdigest.blogspot.com/2013/06/chimpanzee-enrichment.html