Beth,
I would start in your own backyard so to speak. I would contact the USGS in Denver and let them know what you are doing. I know they have educational information on their web site and I bet they might be willing to help a bright young high school student. I would also check out the Colorado School of Geology ad see what they might suggest. Reading through the suggestions I see many great places. Grand Canyon is always good especially if you hike or ride to the bottom. Been there 7 times and each one was different. Another might be the Barringer Meteor Crater in Winslow, Arizona. If you get into Idaho, don’t forget to go to Kellogg and see the Coeur d’Alene Mining district. There is a mine called Crystal Gold Mine open to the public. Might be touristy but still is interesting to go underground and get a feel of the earth from the inside! I think there are many state Geology guides that give self guided trips to various locals and explain things as you drive from point to point. I know there are several for California. Might be able to find something on line. If you get to L. A. a must is the La Brea Tar pits. Great collection.
Whatever you do, have your daughter take many pictures and keep a running diary of your adventures. Collect as much material as you can. Never know when it will work right into a paper or what have you!
Good luck and Happy exploring!
Bob in Woodland Hills