You can hike this as a loop with one portion above ground, the other below ground inside the lava tube cave. Bring a light or rent a lantern at the ranger facility at the trailhead (be sure to check seasonal hours before you head out). The hike inside the cave can be a little tricky so for the less adventurous there's a half mile out and back option underground. The above ground portion of the hike crosses exposed lava flows and offers glimpses of Mt. St. Helens above.
From I-5, take WA 503 East toward Mt. St. Helens. Follow the signs to Trail of Two Forests / Ape Cave.
http://www.tripleblaze.com/trail.php?c=3&i=2037