I'm assuming while at Hat Creek you visited Lassen NP? Geologically interesting and there are hot springs and thermal areas that are somewhat akin to parts of Yellowstone.
Further north is
Lava Beds NM. If she is much of an explorer, she would really get a kick out of the lava tubes. You get to explore many of them on your own. Take a couple of flash lights or head lamps. The NM also encompasses the area where the Modoc War took place. History can be interesting when one travels to where the events took place.
Others have mentioned the coast. Besides those places in CA, you could consider heading north into OR. Harris Beach SP has FHU. From there, she can explore the tide pools. Check out
this brochure for ideas on where to go and what to see. When we have travelled to the coast, I have found that in many places, some of the lowest tides of the year are around the summer equinox in June. The lowest tides means the deeper pools with more interesting life forms are more easily accessed. You can also head south and do some exploration in the Redwoods. Also, you can head a little north and tour one of the
lighthouses.Of course there's Crater Lake. There's a hike down to the lake where you can take a cruise around the lake or get dropped off on Wizard Island for a few hours.
Also in OR is Oregon Caves NM. The caves are interesting for most kids, but the Lodge alone is worth the visit. It's one of those old chalet style lodges. It actually has a stream running through the lodge! What kid wouldn't find that cool. (I know I shouldn't call a 16 y.o. a "kid", but I call people a lot older than that kids anymnore -- I remember my grandmother saying the same thing!)
