Beirise, J. H., Reinhardt, V. 1992. Three inexpensive environmental enrichment options for group-housed Macaca mulatta. Laboratory Primate Newsletter 31(1), 7-8.

We distributed the following enrichment materials on the floor once a week, each on a different day: (1) 1 kg roasted peanuts in their shells; (2) 32 ears of hard corn; (3) one non-corrugated cardboard box. ... After a habituation period of 8 weeks, [2-hour] behavioral observations were made. ...The corn was the most effective eliciter of foraging activity, engaging the animals about 77% of the time. Next in effectiveness was the box (65%) and finally the peanuts (47%).