Our family enjoys learning while we explore. I suppose our kids don't have much of a choice since wife and I are both educators ;-)
We have thoroughly enjoyed doing the Junior Ranger programs at each National Park or Monument we visit. Some of the recent programs we have done include Rainier, Olympic, Theodore Roosevelt, Glacier, and Rushmore.
Washington State Parks also has a program. We've done Alta Lake and Sun Lakes / Dry Falls.
Recently we discovered that Metro Parks Tacoma has a Junior Ranger Program at Point Defiance Park. For any fellow South Sounders, you may want to check it out.
It's not only fun to help the kids with the activities in the books and earn badges - we find as adults that we "see" things differently and learn a great deal about ecology, culture, geology, and history. Good stuff.