Yellowstone has a great junior ranger program they also have a young scientist kit that you can reserve. The kids had a great time running around pointing the heat gun into all the hot pools and geysers to find out the temperatures. Then we could figure out why some were hotter than others and observe the colors and depth. The kids entertained themselves by running in the woods by the campsite pretending to be buffalo.
Another favorite source of good dirty entertainment has been our sand box.
Everywhere we have moved we have built a sand box. Hours of building together and playing has been done in the sand box. The kids work together to make cities, ovens and food, rivers and mountains. My youngest one loves to recreate places we have visited.
Grandpa uses his tractor to till his sandbox and create a huge hill. The sand hill is always a favorite during every reunion. We finally have a sand box big enough, the house we bought in Utah had a sand volley ball court!! We have not yet volleyball in it but many, many hours have been spent in the sand.
Our fascination with starfish started when we lived in Portland OR. The beach was less than an hour away and made the perfect spot for a weekend drive.
At the time you were able to collect starfish from the rocks. We were captivated by them how they ate, how strong they were and all those tiny suckers.