I haven't ever been to Yellowstone (but hope to get there with my kids :))... but fwiw would add another plug for the Jr. Ranger program - our kids have done it at quite a few other NPs and loved it when they were younger
Some of the NPs have an additional program too - can't remember what it's called, but it includes more science experiment types of things - materials included along with instructions. I have fond memories of testing the ph level of water samples on one of our vacations with ph strips that had obviously outlived their life span - and not being able to convince my kids that the issue was the strips
Have fun!
polarbear
