We chose the Bells Mountain Trail which starts at Moulton Falls and connects with the Tarbell Trail to reach Rock Creek Campground. We left the trailhead on Friday afternoon, joined by some of my brothers (I come from a large family), at about 3:00PM and reached Rock Creek Campground at 7:45PM. Distance was 11.3 miles (we made a wrong turn and ended up at Cold Creek Campground), average speed 2.5mph. We spent the night in one of campground lots (no fees for these, also no amenities), and hit the trail at about 7:30AM on Saturday morning. We covered the 9.7 miles back to the car in under four hours, reaching the car at 11:15AM, 2.7mph average. 21 miles covered in ~20 hours.
The first few miles of Bells Mountain Trail are fairly steep and a good workout. Coming back they were a bit hard on the knees but overall I'd say the trail is fairly easy. The entire trail is very well maintained and is sometimes used by mountain bikers. The route was fairly scenic, with great views of Silver Star, wildflowers in the clearcut logging areas, and the occasional glimpse of Mt. St. Helens. Here are a bunch of photos...
All the photography in the post is mine, I use a Canon 7D and various lenses rented or owned. One of my brothers (Benj) is also a photographer and he shoots with almost the same gear. DSLR cameras are a lot of gear to hike or climb with but the results are usually worth the extra weight over a point-and-shoot, if you're into photography. I carry a GorillaPod mini-tripod or a sub-2-lb standard tripod. 