For something strenuous, follow the 2.5-mile Norway Pass Trail (No. 1) through blown-down forests to the 4,500-foot pass, gaining 900 feet in elevation along the way. You can continue beyond to the Coldwater Lake [1] area, return the same way you came, or return to Independence Pass along Road 99 via Independence Pass Trail (No. 227), with extraordinary views over Spirit Lake. The round-trip distance for this is six miles. The Norway Pass Trailhead is just a mile north of Meta Lake; a water pump is also here.
Harmony Trail drops 570 feet in just a mile, offering the only legal access to the shore of Spirit Lake. Most living things were killed when pyroclastic flows filled it in 1980, causing water temperatures to reach almost 100°F. The trailhead is halfway between the Independence Pass trailhead and Windy Ridge [2].
At Windy Ridge you can hike the six-mile Truman Trail (No. 207) for up-close-and-personal views of the lava dome inside the crater, and to connect with the round-the-mountain Loowit Trail (No. 216). A spur of this (216 E) leads to Loowit Falls, where 100°F water pours out of the crater (no swimming).
Links:
[1] http://www.moon.com/destinations/washington/south-cascades/mount-st-helens-national-volcanic-monument/west-side-ac/coldwater-ridge
[2] http://www.moon.com/destinations/washington/south-cascades/mount-st-helens-national-volcanic-monument/east-side-ac/windy-ridge