I decided to hop on this trail to stretch my back after hauling supplies up/down the adjacent High Rock trail, in support of the Lookout Tower renovation project. Perhaps the High Rock volunteer work fatique influenced this review as this Greenwood Trail falls into my one n' done category.
Pros:
1.Arguably the best combined view of Mt Rainier with High Rock Lookout Tower (pic 1). The perspective and juxtaposition resonated with me. A few hours prior I had been up at the tower looking out at where this pic was taken.
2.It shares a trailhead location with High Rock, so you have options.
Cons:
1.Much of the trail has over-grown vegetation. Though the trail is easy to follow, the vegetation forced me into a slow pace because I could not easily see what was below my feet.
2.Greenwood Lake is not an inspiring destination. Small lake bed had low water level (summer drought) and mucky shoreline. "Nothing to see here."
3.Trail is popular w/motocross riders. Nothing say's let's share the trail in peaceful harmony better than having to scurry up into the brush so motorcycles can roar past. Although, they did open a path through some of that over-grown vegetation...
4.The 9-mile stretch of road driving up to the TH is rough. Take it slow to navigate around the potholes.

Excellent hike!! Lake is beautiful and offered great trout fishing. I entered via the High Rock tower trailhead and it was a solid 4.5 miles to the lake (despite what WTA or Alltrails says). Some snow along the last 1/2 mile, but completely manageable. Definitely a hidden gem, the High rock trailhead overshadows the Greenwood Lake trailhead and I was the only person on the trail all day. Dont skip this one!

Opted to hike in the opposite direction of the masses heading up High Rock. Trail was quiet and enjoyable. Patches of snow that are mostly manageable until you get ½ mile from the lake. Decided to turn around because it got too sketchy. Nice spot at 46°39'14" N 121°53'43" W to have lunch on the way with views of the mountain.

We were aiming for a hike on Labor Day weekend that wouldn't be crowded and we found it! After the trailhead, we didn't see a single person! This trail starts across the road from High Rock Lookout. It starts out on an overgrown, unused road then heads downhill on a narrow track for quite a ways. The rest of the way is basically uphill until the last half mile which is gentle switchbacks downhill to the lake. We all had trekking poles which were extremely helpful. At the split you meet a road where you can turn left for Allen Mtn. Or turn right on another narrow trail for the lake. There is a small sign to mark the trail here, but no name on it. I printed off a map of the area from the Forest Service ahead of time.

The lake was quiet, peaceful and mostly bug free! The kids had fun climbing on the logs. They caught a few tiny frogs to look at too. I tried to fish, but I didn't have time to wander. I saw a few small brookies, but they didn't want what I had.

The berries along the second half of the trail were abundant and ripe. We saw salmon berries, a few black raspberries, a few thimbleberries, huckleberries maybe?, and blueberries. This was a great morale booster for the kids.

Speaking of the kids... they did great! I was worried about the sections that were steeping had dropoffs along the side, but they listened to directions and took it slow. 

Overall, I think I'd prefer this if we were going to stay overnight. There was one large area for a campsite. It was a long hike for one day. 

5.6 miles and 5 1/2 hours