In his article "Chimpanzee Enrichment", zookeper Peter Dickinson gives a nice introduction to the concept of Environmental Enrichment for zoo animals and makes a nice list of tools and ideas.

In the introduction to the article, he writes:

Where two or more chimpanzees are kept together they can, will, and do, invent and implement their own entertainment and games and “enrich” their lives. The more animals there are, the wider the span of ages and sexes the greater the number of scenarios and intrigues. This is natural behavior, ‘chimpanzee politics’ in the raw. Their daily entertainment can be vastly expanded by what is made available to them in their surroundings [...] One of the most important animal husbandry duties is for carers to provide “Environmental Enrichment” to their charges. Although a relatively new term, it is something which good animal keepers have been doing, when they had a minute, since the Ark [...] Official recognition of the existence and the importance of enrichment has been slow in coming. Some zoos today have staff, or even whole departments dedicated to this intriguing mix of art and science [...] Enrichment needs time for planning, for constructing, for implementing as well as for observing and recording! Enrichment is not an Extra, it should be an integral part of daily animal husbandry.

There is a long list of tools and toys he is experimenting with - for example cloth, PVC pipes, pets, radios, mirrors and so on. Chimpanzees seem to get bored soon so it is important to change the offerings quite often, always trying to stimulate another sense: 

One should try to enrich all the senses.

Since we work with birds, hearing and sound are the main sennse to stimulate, of course. But we shall not forget any other aspects, like motion, appearance, tactility ... In this aspect, Mr. Dickinsons article is an important source of inspiration to us!

Read the article online here:

http://www.australasianzookeeping.org/Husbandry%20Manuals/Chimpanzee%20Enrichment.pdf