At first glance, it would seem very little, as most are too small to be ridden, but the larger minis may be ridden provided the child isn't too large. Saddles and bridles for minis can be purchased for this purpose. Most minis are very friendly and outgoing animals and make great pets. They can be trained to pull a cart, as already mentioned. They make excellent harness ponies, and can be easily handled by children and adults alike.
Some people take their minis to schools and make presentations to the classes.
We have done this with Clover. Some also use them as Therapy animals at nursing homes and hospices. And of course, there is always the show ring. There are halter classes where the animal's conformation is judged, driving classes with carts and small carriages, and also trail classes with obstacles that the mini must maneuver around, through, or over. There are also jumping classes in which the mini is led over a series of jumps.