Webmomom,
Both are free. You will have to check for the time of the Lobby Tour when you arrive (ours was at 3:30 each day). If you inquire at Guest Services they will be able to direct you to your guide. My DW and I were the only guests during our tour. Our tour guide, Jed, was a young man from South Africa who gave us some great information on the artifacts and structure of the lodge. The tour lasted about an hour and was very interesting and informative. The CM's who give the tours, answer questions at the savannah overlooks and work in the restaurants are mostly from countries in Africa. They come over here on 1-year visas as part of an exchange program and are very interesting to talk to. Don't be shy; ask questions!
The Hidden Mickeys Safari is also fun. Ask for a clue sheet at Guest Services. Ours had 17 clues for Hidden Mickeys to be found around the entrance, lobby area and Arusha Rock area of the resort. We found them all (with a little help on a couple) and were given a certificate signed by Mickey and a nice gift.
We did not attend any of the story telling sessions at the Arusha Rock fire pit but plan to do so on another visit. They are also free.
I hope this answers your questions.
Mike