This is a fantastic trek which I recommend very highly. It has everything - gorgeous rain forest scenery and a beautiful river all along a fairly easy trail which is easy to access.
From the parking lot, you cross the little footbridge and start walking up the concrete road into the hills. You have to walk about two kilometers to reach the trailhead, but you pass by a pretty river along the way, so it's not a hardship.
The trail itself is mostly dirt and crushed stone heading uphill at a gentle slope the entire way. At one point you have to cross over a river along a rope bridge, which is fun. And in a couple of places you have to walk over large rocks. The rocks can be kind of slimy, so you should wear good walking shoes.
The point is just to admire the beautiful forest and river scenery. There is an amazing array of plant species here - this is what scientists mean when they talk about biodiversity. In particular, there are lots of birds nest ferns growing on the branches of trees, which are a wonderful sight.
From the parking lot to the top of the trail is a little more than two kilometers. Walking up and back takes about two hours if you keep moving, but I recommend that you take your time and savor the gorgeous scenery. The forest here is incredibly photogenic, so be sure to bring your camera. (I'm lazy so I just take pictures with my iphone - they don't do the scenery justice.) This place is paradise for nature photographers.
The only problem with this place is that it's very popular. The parking lot is huge, which is a bad sign. I visited on a warm Monday in November and I only saw two other people on the trail. But I bet there are a lot of people on weekends during the high season. So if at all possible, visit on a weekday.
To get here, take Road 7 west from Yilan City and drive up into the mountains. You will pass a long bridge over the river, with a nearby gas station. Soon after you will come to the town of Songluo. The trip from downtown Yilan City to Songluo takes about half an hour. This is an aboriginal area - people here belong to the Atayal tribe, so the trail is marked by a big wooden sign with an aboriginal warrior next to the side of the main road. The access road to the trail is on the left side of the town, next to the big wooden sign. Turn right here and follow the access road down to the parking lot.
After you've walked the trail, if you have the time I recommend you swing by the nearby Cherry Blossom hot spring (see my post) for a bath. Soaking in the hot water feels wonderful after the hike. To get to the hot spring, get back on Road 7 and keep going west (away from Yilan City). After about 5 minutes you will get to a bridge over the river - the Douniu Bridge 鬥牛橋, with a sign pointing the way to Douniu. Cross the bridge and drive for another 5 minutes. The hot spring is marked by a hot pink sign on the right.