We took many family vacations on Lake Almanor near Lassen and Chester. Great family times! We hiked to the top of Lassen three times and I always enjoyed the hike up. We always had good outdoor gear and footwear...I remember idiots hiking up in flip-flops and no water! You know how hot it gets there in late August. I was always amazed at the idiots out on hikes.
I took my older daughter one year to "Cinder Cone," a small mountain of volcanic cinders with a crate at the top. It was the most unusual geological feature I've seen. You had to walk by the front edge of an enormous lava field to get to the cone. Half way up this bone-dry, desolate cinder cone, my daughter found this frog! It was the weirdest thing ever. How did he get there?
Here's the huge lava field. You can see my daughter at the base of it for scale.
I backpacked in Lassen twice with roommates. We did the big loop which took three or four days. It’s interesting to compare the areas which are heavily forested with those which are barren or where the forest has just begun to come back after an eruption. We didn’t try to climb any peaks. Definitely you need to take water. Very beautiful park.