Falls Creek Trail is a good one for introductory backpacking trips, or simply a long, woodsy day hike through the forest. You'll enjoy a gradual grade of this 9 mile trail. Hike the full-length (making for an 18-mile trip) or turn around at any point.
The first couple of miles are likely to be your most crowded, since many people use Lower Falls Creek as the other half of a 6-mile loop hike with a waterfall in the middle. But once you've passed the waterfall (about 2.5 miles from the trailhead) you'll find the hikers fewer and further between.
The trail continues on, bordering Falls Creek to your right, until 3.8 miles from the trailhead, it joins an old road bed for a bit more than a mile, before returning to true trail and then crossing Road 67, 5.7 miles from the trailhead. Continuing on, it's just one more mile to the Lava Caves, remnants from Mount St. Helens' eruption.
Here many hikers may want to call it a day, but Trail 152 does continue on, terminating at Falls Creek Horse Camp on Forest Road 65.