Check the FCO site for Malawi, you will find that you cannot work in Malawi, even voluntarily, unpaid, on a tourist visa, so it will be necessary to contact the Malawi Embassy to find out what is required. This type of ruling may apply in other countries. African elephants are very rarely close enough to humans to feed and ride. This is something that is usually seen in Asia. There is an elephant orphanage in Kenya where it is possible to get close to young elephants. In Malawi there are plenty of childrens homes where help would be welcome,as are there many non government hospitals which rely on various charity/religous/NGO organisations. This all depends on your qualifications. I suspect some of these so called voluntary organisations are little more than fronts for holiday firms. Some of the work that they do does more harm than good,often the project peters out, and the people are left where they began, its no good supplying a water pump if there is no follow up to keep the system working.

Malawi is a great country, with great people, who need a great deal of help. It used to be by far the safest place in Africa, and although thing have changed, it's still the best, as long as you take sensible precautions appropriate for the area you are in.