This US Forest Service site near Bend Oregon has a long lava tube cave, obsidian, lakes, a view of the volcanic Cascades above and more. 7,000 years ago a lava flow redirected the Deschutes River which now runs along the north part of the monument, and there’s an easy hike to Benham Falls on the river trail. The open season is quite short in summer, with restrictions on visitors to different sections. With the heat these days, the monument should open by May 1st, but whatever. I arrived early before other sections opened in June, and went to see the lava cast forest. I’ve never seen so many clear impressions of tree trunks left behind after being surrounded by lava. See one example below.