Compared to most national parks, Lassen Park is uncrowded and unspoiled. It has many beautiful family friendly and more strenuous hikes from both the east and west sides. The new Kohm Ya Ma nee Visitor Center off highway 36 (about an hour east of Red Bluff) is a good place to start. It has interactive exhibits that the kids will love and an excellent movie that explains the park's volcanic history. In Lassen, there are several areas of fumaroles and hot spots. The most crowded is Bumpass Hell, off of the main park highway 89. This highway frequently doesn't open until July due to heavy winter snow. Another volcanic area is available with only an hour or so easy hike through picture-perfect meadows. This area is called Boiling HotSprings and you can reach it through the eastern end of the park/Warner Valley. We've taken hikes here in July and August where we have had great hikes and encountered relatively few other hiking parties. You can check ahead with Drakesbad Guest Ranch and make a reservation for dinner....
Although the drive into the Warner Valley is mostly dirt road, it is worth the time and trouble.