Lassen Park is only open for 2-3 months a year. Because of its elevation of 2000+ meters and location, the snow doesn't melt until around July. Right now it's only +10°C at night, though it's still warm during the day. But soon it'll get really cold. About 0.5M people pass through the park each year (compare that to Yosemite — 4M). That's why it's easier to spot wildlife here: the human trails aren't as heavily trafficked.

The park is one of the few places in the world that has volcanoes of all four types. And with volcanoes comes plenty of geysers, sulfur springs, and other boiling lakes. I'll send photos this evening.