If your looking for a moderatly strenous day hike, Lena Lake in the Olympic National Park is a good trail to travel. The trail is well maintained, and easy to follow. It is about 3.5 miles from the trailhead to the Lena Lake Campground, with about 1,600 feet elevation gain.
The weather was great, no rain, lots of sun and very warm. The Lena Lake trail begins with a series of long switchback (I believe 9 in all) starting in second growth forest but soon blends into old growth forest with large trees, and fallen underbrush heavily overgrown with ferns and moss. There are two sturdy bridges the first crosses a dry riverbed and the second crosses a gentle cascading creek.
Lena Lake is a good sized lake surrounded by peaks, trees and peaceful silence. If you are planning on more than a day trip, there are several nice campsites near its banks.
Although the trip is about a 7-8 mile round-trip hike, I would plan on a full day to give plenty of time to enjoy the walk through the lush forest and the lake. It is mostly up hill on the way in, and all down hill for the return trip. Regardless, this hike makes for a day well spent.