The highlights of Lassen are the volcanoes and their offshoots: boiling springs, fumaroles, mud pots, and so on. You can see many of the most interesting sites in a day, making it possible to visit Lassen as a short detour from I-5 or U.S. 395 on the way to or from Oregon.
Bumpass Hell, a 1.5-mile walk off the main road in the southern part of the park, is the largest hydrothermal site in the park -- 16 acres of bubbling mud pots cloaked in a stench of rotten egg-smelling sulfur. The colorful name comes from an early Lassen area homesteader guide, K. V. Bumpass, who suffered a severe burn on his leg after falling though thin earth into boiling mud during one of his tours. Don't make the same error.
Sulphur Works is another stinky, steamy example of Lassen's residual heat. Two miles from the southwest park entrance, the ground hisses with seething gases escaping from the ground.
Boiling Springs Lake and Devils Kitchen are two of the more remote hydrothermal sites; they're in the Warner Valley section of the park, usually reached from the Feather River Drive/Warner Valley Road near the small town of Chester.