Local organizations
Depending on where you live, there are different possibilities. In the Vancouver area, there are many choices - most of which are reasonably priced. For example, kids can do camps at local parks (through the Park Board and community centres), with the botanic garden, aquarium, museums, and so on. For an animal focus, most animal shelters and wildlife groups offer some kind of hands-on or educational opportunity. If you live near a larger park - state/provincial/national - they may have a volunteer program or camp. Also, many traditional summer camps (sleep-away) also have an outside or environmental focus. Try googling "environmental camp" and your community.