Choose several that have interesting rhythms and put them into an order within a phrase. Add an ostinato pattern to this and it will instantly become even more interesting. Canons, Rounds and Partner songs are excellent, obvious ways to work on vocal training and various musical concepts. A variation on this type of work is the use of vocal ostinato patterns. Layers these together to create a musical experience.
Body percussion is also effective for improvisational exercises.
Water cooler jugs, buckets, and cardboard boxes make satisfying drum sounds as well. Fast food restaurants usually have wonderful large plastic pickle barrels to give away for a conga type sound. Various shakers can be created with assorted bottles full of rice, beans or gravel. Tin cans also work well as quiros. Old keys, suspended one by one from a stick, can make an inexpensive version of a chime tree.