With the age difference between the kids, I think it's good to find things that can be shared but provide different levels of skill, etc.
Guide books that they can check off as they ID various birds, butterflies, trees, etc. The older one can read the descriptions, the 3 year old can help identify features (yellow legs, long thin beak, etc.), and the 1 year old can look at the pictures.
How about photography? You could get the 9 year old and 3 year old inexpensive used digital cameras and let them have at it. The 9 year old can explore a bit further (maybe take a hike with one parent while the other stays at camp with the little ones). The 3 year old will just enjoy running around the campsite taking photos. The older one can find lizards, bugs, etc. and take photos of them for the little ones to see. You can also give the older one "assignments" with the camera. Things like take a photo of wood. It lets the kid use his/her imagination as to what "wood" is - maybe a tree, bench, sign, fence, twig, etc.
Bringing a friend for the 9 year old could be nice also. Sometimes the hardest thing about the age difference between the kids is that the oldest one can be restricted to playing alone or playing "baby games" - a friend along can provide some age-appropriate fun.
