We've all been witness to, and some of us may be to blame for, the red boomer balls in tiger exhibits or the blue barrels in the polar bears habitat. I've heard guests laugh about beer kegs in the bear exhibits, implying in some manner that the bear's an alcoholic. Positively enjoyable for the guests and the bear, but still a problem as they bring a wholly artificial element into an otherwise "natural" setting.
In an effort to curb these disruptions in our suspension of disbelief in an immersive zoo exhibit (in other words, in order for us to get rid of any sign that we are, in fact, in a zoo, and not in Borneo or Alaska), we need to start planning the enrichment as a part of the exhibit design process with the keepers.
Jon Coe wrote a nicely illustrated paper for the Australasian Regional Association of Zoological Parks and Aquaria conference back in 2006. Within this paper not only does he clearly outline several concepts for enrichment devices within exhibits, but also lays out some general guidelines. Take a look!