There could be confusion over which gibbon is being enriched, and why the food isn't released if the 'wrong' animal stands on the pad. To avoid this, there could be 2 feeding stations, perhaps in different locations, one for father and one for son, and 2 corresponding light signals. Would the gibbons understand the connections and only respond to the signal directed towards them specifically? Alternatively, they could be fed simultaneously, but from different food-drop sources, and if dad is busy standing on his pad, son will be free to stand on his own one without anyone trying to take his food...
Instead of tokens and slot machines distributing treats, useful tools (sticks, sponges, rocks etc) could be menu items that are found in specific locations. Resources such as water should be available in at least 2 different places, so everyone has a chance to collect and no-one can dominate 2 places at once. Ideally, the route between interesting locations (for tools or food) should be ci rcular, so it's possible to avoid other individuals and never be trapped in one place. Staff should be able to block parts of the route if they need everyone to come indoors at night.
The bridge between chimps and visitors could take the form of a parallel route for humans, who might like to test their reaction time and speed against chimp reaction time and speed (menu screen lights up - banana drop HERE!)...