My wife and I made our annual fall pilgrimage to Yosemite last weekend. After enjoying afternoon sunshine and scenery in the meadow east of Yosemite Lodge, we ran across the common sight of a small group of people on one of the ranger walks. Much to our delight, it was being led by NPS "celebrity" ranger Shelton Johnson, familiar for his contributions to documentaries like the Ken Burns national park series and his personal projects recognizing the Buffalo Soldiers. Of course we joined the group and were treated to 1 1/2 hours of detailed and totally engaging commentary. The topic was "bears", but it expanded to include a much broader perspective of experiencing and appreciating nature. His knowledge and passion made for a rare and thoroughly enjoyable afternoon, one that ended much too soon. I don't know if you could get information on when he does this sort of thing (it was just listed as the afternoon ranger walk) but it would be well worth the effort to try. It added a real highpoint to our collection of Yosemite experiences.
In case you aren't sure who Shelton is: https://www.youtube.com/watch?v=7A1GiSnkBc8