The Lassen Peak trail is a very steep, switched-back trail that gets lots of travel in summer. There are good views of the surrounding volcanic park and Mt Shasta from the often crowded 10,400 ft summit.
If you are into adventure, the summit of Lassen makes a worthy ski trip when conditions allow. The south entrance is open to the old ski lodge parking and chalet in the winter giving access to ski or snowshoe trips off of the unplowed road to Sulpher Works, Lake Helen, and beyond for all ability levels. Remember, winter can be a great time to visit uncrowded National Parks!
I agree with the above. on the park in general. I have only hiked to the top once but it was an experience my father and I will never forget. We just took our daughter up this last winter and for a little one just learning to walk she had a lot of fun.