Lava Beds National Monument is in northeast California.  We visited it as a day trip while we were staying in Weed for a few weeks.  It's a long drive there and back, but we enjoyed it.  The key feature there is the lava tubes formed when the lava flowed over this area.  There are other features having to do with human history but we didn't have time to view them.

We went to the Visitor Center, then walked through three lava tubes.  The one near the visitor center is lighted and paved, and it is the only one that is, although others have some structure like stairs.  We chose three of the least challenging caves, and found that was all we could do in a few hours. 

The Mushpot cave is near the visitor center and is paved and lighted.  Its signs educate you on what these lava tube caves are all about.  Then we visited the Sentinel cave which has separate entrance and exit and no stooping.  We then visited the Skull cave which has stairs down to the bottom where ice forms year round.

Unlike many caves you see, all features in the caves here were caused by lava as it flowed through and out of the tubes.
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We hope to go back for a more extended visit.  Highly recommended especially for geology buffs.