Busy Beavers here
I love everything about Mount Lassen including the Manzanita Lake camping ground.
UNFORTUNATELY the lake allows no water Recreation due to a over population of beavers; Beavers are apparently dangerous.
It is located less than a mile from the entrance. Large and well shaded sites. Very affordable!
All the amenities including gas, laundry and showers.
Stock up on supplies as the store is limited and expensive!