The first hike we did in Mt. St. Helens National Volcanic Monument was a short, 2-mile loop hike in Lava Canyon. This hike begins with a wide, paved, wheelchair accessible Lava Canyon Interpretive Trail, which leads to a viewing platform overlooking the canyon and the Muddy River that cuts through it. From the viewing area, the trail loops down to the canyon rim in a long series of switchbacks. At the end of the switchbacks there is another viewing point, which offers good views of waterfalls on the Muddy River.
Soon afterward, the trail crosses a springy suspension bridge, which can give a bit of a scare to people with a fear of heights (like me). After crossing the bridge, you can either loop back to the trailhead, or you can continue into the gorge–a steep descent which includes climbing down a 30-foot ladder. We opted for heading back to our car, but I'm sure the views would have gotten even better if we had continued down the canyon. Still, even the short version of the hike was very enjoyable.
Lava Canyon and the Muddy River: