Devil's Postpile: a great big pile of columnar basalts (60 feet tall!)
Mt Lassen: after hiking it and Bumpass Hell, we got a stellar view of the peak (a volcanic plug dome that last erupted in 1914) as we exited the park.
Also in Lassen Volcanic National Park was a trail to "Bumpass Hell" - yes - the hell was entirely because of the stinkyness. BOY did it reek! I took video of the hissing fumaroles, mudpots, etc. - one of them is here. You are lucky that there is no "smell-o-vision" available.
Clearly showing the haze of the smoke from the forest fires, this photo (taken from the top of Cinder Cone, a cinder cone volcano in Lassen Park) also shows the Painted Dunes and Fantastic Lava Beds - I can only imagine how glorious the scene is where there is no smoke...
Mono Lake - a place I'd actually been to before - here are tufa pillars poking out of the alkaline lake. They are formed from hot spring (volcanic) activity beneath the lake and the mixing of waters resulting in calcite (calcium carbonate) deposits.