Way too many must sees to explain.
Some people enjoy the museums, some enjoy the touristy places like Confusion Hill, or the Dinosaur Cage. Some prefer just enjoying the natural beauty. That is why we always attempt at making the journey a major part of the trip.
You can do a search in finding some of the many scenic routes. The freeway systems here are a fast way to get from point A to point B but we stay off of them as much as possible. The secondary highways are a good way to go.
An example: Hwy 89 connecting Lassen Park and Burney Falls. Beautiful country. Enjoy